Unaware of each other’s presence, Donna Noble and the Doctor both make their way to the Adipose Industries building in London. Both claim to be health and safety inspectors but their mode of entering the building differs – whereas Donna bluffs her way through security, the Doctor breaks in through the back door.
Soon they are both in a lecture theatre, the Doctor observing from the projection room at the back whilst Donna sits in the stalls. They listen as Miss Foster, head of Adipose Industries, introduces to her audience the company’s main export – diet pills. She tells them that one pill taken daily for three weeks will completely removed excess fat, which she claims will simply “walk away”.
A reporter names Penny Carter interrupts the talk several times to air her scepticism of the company and its product. Miss Foster dismisses her, claiming the company have one million customers in Greater London alone, and are soon going to market the pills nationwide.
After the talk Donna and the Doctor, still ignorant of the fact the other is in the building, make their way to the sales area and station themselves within two separate worker’s cubicles. They discover that each client of the company is rewarded with a solid gold Adipose Industries pendant, an example of which they each decide to take for further examination before asking the members of staff they have intercepted to print them off a list of the company’s customers. During this time Miss Foster arrives to threaten the staff with removal from the firm if their sales records do not improve.
That night Donna visits Stacey Campbell, one of Adipose’s customers. Likewise the Doctor calls at the house of a man named Roger Daley. Each pair discuss the weight loss pills and both clients appear to be happy (and visibly thinner) having used the product. The Doctor discovers that Roger has periodically lost 1 kilo of weight after taking each pill, and that it appears the weight vanishes at the same time each night. Roger explains he has discovered this because every night he is woken up at the same time by his burglar alarm going off, apparently for no reason at all. Once woken, he takes the opportunity to weigh himself and discovers the excess fat has gone. The Doctor theorises that the house’s cat flap may be the cause of the alarm sounding, remarking that things can not only get into the house via the flap, but they can also leave. When Roger asks what he is hinting at the Doctor recites Miss Foster’s sales slogan: “The fat just walks away”.
Back at Stacey’s home Donna waits in the living room whilst her new acquaintance goes to the bathroom. She takes the opportunity to examine the pendant she took earlier and discovers the small gold capsule at the end of the chain is made up of two separate sections, which can be twisted. She tries this several times, unaware of its effects. Up in the bathroom Stacey begins to feel ill, and her skin begins to move to the point where a bulge appears at her stomach. Back at the Adipose Industries building an alarm sounds and Miss Foster announces that premature Parthenogenesis is taking place somewhere in the city.
The Doctor leaves Roger’s house and also detects strange activity occurring in the metropolis, tracing it using a small botched tracking device. Miss Foster, meanwhile, orders her ‘Collection Squad’ to go to the source of the disturbance and “bring them home”.
Meanwhile, Stacey’s stomach continues to expand until the lump forming there breaks away from her body and falls in the sink. She looks down to discover it is a small white creature, which waves to her. Back in her office, Miss Foster claims that “the Adipose has been witnessed”, and so she must now activate full parthenogenesis. She takes out her own pendant and twists the small capsule hanging from the chain.
As more little creatures beginning forming and splitting from Stacey’s body, Donna, who has realised something is wrong, ascends the stairs and walks over to the bathroom door. Suddenly she hears a scream and behind the door Stacey’s body suddenly disperses into a dozen or so tiny creatures. Donna forces her way inside to find Stacey’s clothes on a pile in the middle of the floor, and then witnesses one of the creatures as it escapes via a window.
Outside, Miss Foster’s hired hands arrive to collect the miniature creatures whilst the Doctor makes his way toward the house using his tracker. Donna rushes outside to look for the creatures but finds they are all gone, collected in the van, which speeds away nearly running over the Doctor. Both the Time Lord and Donna continue to search the streets, again uninformed that the other is simply metres away from them.
Meanwhile, Miss Foster realises that one of the pendants has been stolen, meaning someone is trying to interfere with her plans. She scans through videos of security footage taken that day then stops when she discovers a female on screen she believes to be the culprit. She then ponders what must be done to stop her.
Donna arrives home and is given a frosty reception by her mum Sylvia, who is unimpressed with her late arrival. Throughout the evening she sits at the kitchen table whilst her mother lectures her about her life, claiming it is time she found a job and moved out again. Although she admits Donna claims she has plans for her life, she accuses her of never making them clear.
Tired of this, Donna makes her way to a nearby hill where her grandfather Wilf is sitting by a potting shed, gazing at the stars with his telescope. She sits down beside him and the two begin to talk, Wilf noting that lately Donna appears to be somewhat distant. She tells him she is waiting for the right man, someone she met once but let slip away from her, and who she now wants back. She tells him to keep a look out for a small blue box flying through the night sky, and to call her when he sees it for it is the box, and the man inside, that she is looking for. She worries that she may never find him again, but Wilf encourages her to keep searching. She agrees:
“You’re right. ‘Cos he’s still out there, somewhere. And I’ll find him gramps, even if I have to wait a hundred years…I’ll find him.”
In the TARDIS console room the Doctor examines the Adipose Industries pendant he took from the office, and begins talking excitedly about it. However, he soon realises that no one is around to listen to him and he ceases to speak, instead choosing to stand alone beside the console.
The next day Donna, against her mother’s wishes, takes her car and returns to Adipose Industries, unaware that the TARDIS lands metres away a few moments later. Again she bluffs her way inside whilst the Doctor zaps the back door with his Sonic Screwdriver. However, instead of looking around the two conceal themselves away; Donna hides in a toilet cubicle whilst the Doctor hangs around the service and maintenance areas. Elsewhere in the building Miss Foster and her guards patrol around, looking for their saboteur.
Several hours later the building closes for business and Donna and the Doctor come out from their hiding places, ready to look around again unhindered. Donna is forced to stop when she receives a call from her mother, who wants to use the car to visit a friend of hers who is celebrating losing a lot of weight after taking the Adipose pills. Suddenly the door of the restroom is forced open and as Donna hides back in her cubicle, Miss Foster and her two armed guards enter to locate the woman they have been searching for.
The guards begin knocking each cubicle door down to find the suspect, and Donna grows increasingly nervous. However, she is surprised and relieved when it transpires that she is not the woman Miss Foster is looking for, it is Penny Carter, who is also hiding in a toilet cubicle. The guards grab her and she announces that she has checked all of the data linked with Adipose Industries, determining it as fake. Miss Foster tells her to be silent, and she is taken away.
Meanwhile, the Doctor arrives on the roof and clambers into a motorised cradle attached to two cables, which can be lowered down the side of the building to enable it to be cleaned. As Miss Foster has her guards escort Penny to her office and tie her to a chair, the Doctor begins lowering himself down so that he is outside the office. He leans down into the cart so as not to be seen but listens in on the conversation taking place inside. Meanwhile, in a corridor outside, Donna approaches Miss Foster’s office and crouches down beside the door, also hoping not to be seen.
Miss Foster, claiming Penny will not live to see the true nature of the organisation revealed to the world, decides to explain the true function of the company. The pills, instead of flushing fat into the bloodstream as they claim, actually bind the fat together to form living creatures. She then discusses her name, Foster, and claims it is one she chose with great care. Claiming she is a foster mother, she opens up a drawer in her desk and reveals her children: the small white creatures created from human fat galvanised by the pills – the Adipose.
The Doctor and Donna both stand up to observe the creature, which is identical to the ones created out of Stacey, and are finally reunited when they see each other across the office. The Doctor is somewhat bewildered by the situation but Donna is jubilant at finding the man she has been looking for. In the silent conversation that follows Donna explains that she, like him, has been investigating the company, having heard rumours connected with the company.
Suddenly she realises Miss Foster has seen the two miming to each other across the room. The Doctor motions for Donna to run up to the roof and he follows by ascending in the cable cart. Miss Foster and her guards soon give chase, leaving Penny tied up in the office.
Eventually the Doctor arrives on the roof and runs back inside, where he meets Donna on the stairs. Realising the guards are on their way they sprint back up onto the roof, where Donna explains that she has been looking for the Doctor. Ever since they met her eyes have been opened to the wonders of the universe, and she wants to explore them with him. She began trying to find him by researching alien activity on the Internet (believing he would not be far behind), and amongst other strange goings on such as huge swarms of bees disappearing, she uncovered conspiracy theories surrounding Adipose Industries, so she decided to find out what was going on.
The Doctor clambers back into the cable cart and zaps the controls with the Sonic Screwdriver. Donna follows him but worries that Miss Foster will simply be able to pull them back up again. The Time Lord is sceptical; claiming he has locked the controls, meaning only another sonic device will be able to release them, and it is unlikely Miss Foster will have such a tool.
Unfortunately for him, she does, and after opening the locked doors leading to the roof, Miss Foster begins attacking the controls of the cable cart with her Sonic Pen. Suddenly the cradle plummets toward the ground at an alarming rate, but is forced to stop when the Doctor locks the controls again. He and Donna then begin trying to break a window in order to climb back into the building but Miss Foster orders for the office block to be deadlocked, rendering their efforts useless. She then zaps one of the cables with her Sonic Pen and it snaps, causing one side of the cart to drop. Donna tumbles out and falls, but manages to grab hold of the snapped cable and is left dangling perilously in mid-air.
The Doctor, having managed to remain inside the upturned cart, tries in vain to drag Donna back inside. He watches as Miss Foster attacks the other cable but stops her by shooting her Sonic Pen from her hand with his Screwdriver. He catches the small device as it falls and uses it to open one of the deadlocked windows. He clambers inside and then runs downstairs to a few floors below, where Penny is still in Miss Foster’s office, now watching Donna dangling from the cable. The Doctor opens the window, helps his friend inside and then frees Penny. He and Donna then run off but are stopped in their tracks when Miss Foster and her guards block their path.
The Doctor enquires as to Miss Foster’s real name, and she tells him. Her name is Matron Cofelia of the Five-Straighten Classabindi Nursery Fleet, sent to Earth by her employers, the Adiposian First Family, to cultivate their children after their own breeding planet was lost.
Donna asks why Stacey was completely disintegrated when the Adipose were only meant to be formed from her fat, and Miss Foster explains that in extreme cases the creatures can be fashioned from flesh and bone as well. The Doctor cuts in, claiming what she is doing is against galactic law, and she must be stop or he will intervene. Unimpressed, Miss Foster orders her guards to aim their guns on the pair but the Doctor manages to escape with Donna by aiming his Sonic Screwdriver and newly acquired Sonic Pen at each other, causing a high-pitched whine that distracts the guards.
Miss Foster does not follow the escaping duo; instead she claims she is instigating premature labour. The guards follow her back to her office where they find Penny rummaging through the company files. They tie her up again and Miss Foster resumes her work – opening up a section of the wall to reveal a glowing green control panel.
The Doctor and Donna meanwhile have descended to the lower levels of the office block, where the Doctor hid that morning. He explains that he has discovered an identical panel, part of a computer system running through the whole building. Now he has the Sonic Pen he can override the triple deadlock protecting it and tamper with it.
Upstairs Penny asks what the computer does, and Miss Foster explains it is an Inducer, capable of accelerating the birth of the Adipose creatures. Now that the Doctor has discovered her plans she is worried he will alert the Shadow Proclamation. As such she must act quickly, activating the million Adipose planted in people across the city as soon as she can. As she sets about configuring the controls she sends her guards after the Doctor and Donna, ordering them to shoot on sight.
However, the Doctor manages to reroute the power of the building so that it hits the guards, rendering them unconscious. He then resumes his work trying to reverse Miss Foster’s plans. As he examines the computer Donna asks if he is still alone, and he tells her of his friend Martha Jones, whom he travelled with but ultimately harmed as a result of his dangerous life. She then asks of his other friend Rose, whom he told her about the first time they met, and whom he claims is still lost.
Donna recalls the brief time she spent with him and how she swore she would change her lifestyle as a result of the things he showed her, but ultimately she remained in her same dead-end life. She manages to bring up the idea of travelling with him, and the Time Lord seems somewhat bemused by this unexpected suggestion. Before he can reply the Inducer activates, and across the city a million people begin feeling the ill effects of the Adipose pills, including Sylvia’s friend and the Doctor’s acquaintance Roger. Each victim’s skin begins to ripple, bulge and separate into tiny Adipose creatures, which swarm out into the streets and make their way to the Adipose Industries building; in Roger’s case, the creature escapes through the cat flat before joining it’s siblings outside.
The Doctor realises Miss Foster has instigated emergency parthenogenesis, and as a result every human who took the Adipose tablets will be completely disintegrated like Stacey. He tries to cripple the Inducer by overriding the signal it transmits, which he intends to achieve by linking the pendant he stole into the machine, the result being that the unborn Adipose would return to simply being fat, not living creatures. However, Miss Foster realises what he intends to do so doubles the strength of the signal.
The Time Lord despairs that he will not be able to save the million people about to die, but is himself saved when Donna reveals she too has a pendant, which he can use to double the strength of the override and shut down the Inducer completely. This he does and soon all those suffering from the parthenogenesis are saved. Miss Foster, disgruntled by this, claims she has still managed to convert ten thousand of the creatures, and rejoices as a distant rumbling is heard overhead – her lift home.
Outside a giant spaceship appears in the sky. People rush out into the streets to watch it arrive, whilst inside the Adipose building the Doctor explains to Donna that it is the nursery ship arriving to take the newborn creatures away. Suddenly the Inducer springs to life as the ship’s transmissions are received. The Doctor reads them and realises something is wrong.
“We’re not the ones in trouble now…she is.”
Outside, Miss Foster stands before the crowd of Adipose, promising they are going to be taken home. Sure enough the ship emits giant beams of light and as the Adipose creatures step into them, they begin floating up toward the craft. Miss Foster calls out to be taken as well so she can care for the creatures, and she too begins ascending into the sky.
The Doctor and Donna arrive up on the roof in time to see the Adipose being taken into the ship. Soon they see Miss Foster ascending and the Time Lord tries to urge her to jump onto the roof and out of the levitation field. She scoffs, but the Doctor continues to warn her, explaining that the Adipose parents will no longer need her now they have their children, and that having discovered that breeding their young on Earth is illegal, they will want to get rid of her. A look of realisation dawns on Miss Foster’s face but it is too late, the beam of light lifting her up is shut off and she plummets to the ground, screaming. With all of the Adipose safely onboard the ship departs, leaving Earth behind.
Some time later the police arrive to clear up the mess. The Doctor throws away Miss Foster’s Sonic Pen as he and Donna make their way to the TARDIS. Donna runs over to her nearby car and reveals that she packed for the trip some time ago, and begins unloading luggage from the back of the vehicle. She talks excitedly about travelling but stops when she realises her friend has fallen silent. He claims he is worried that the life he leads will harm her, as it did to Martha. He tells her he doesn’t want things to get complicated again, he simply wants a friend. Donna agrees, somewhat disturbed by the prospect of a relationship with him, and prepares to leave, before realising that she still has her mother’s car keys.
Whilst the Doctor begins loading her baggage into the TARDIS Donna calls her mother on her mobile phone to explain she is staying with a friend for a while. She tells her the car keys are hidden in a bin nearby and sure enough as she hangs up she throws the keys into the container. She then turns to a blond woman standing in a crowd watching the police and asks her to point out the bin when her mother arrives. This done she hurries away, leaving the woman to turn and reveal her identity – it is is Rose. She looks solemnly back at the police blockade then turns and walks away alone, fading from view as she goes. Donna meanwhile returns to the TARDIS and steps inside, where the Doctor is waiting. He asks her where she wishes to go, and she claims she knows exactly where.
Back on the hillside Wilf is still stargazing. He is startled when he sees a small blue box flying past and, remembering what his granddaughter said to him the previous night, calls out to her. He hurries to catch a glimpse of the object in his telescope, and when he does he realises it is a spaceship. In the doorway of the craft is Donna, and standing just behind her is the Doctor. They smile and wave and Wilf cheers them on; glad Donna has found the world she was looking for. As the TARDIS spins off he dances and cheers, happy his granddaughter is where she wants to be at last.