Setting the TARDIS coordinates to random, the Doctor takes Donna to her first alien world. They excitedly step outside the ship and the Doctor eulogises about seeing a new planet, commenting that after a mundane life on Earth Donna finally has a new surrounding. He turns back to the ship, only to find that his companion has returned inside to find a warmer coat. She emerges moments later and together they look on as a rocket soars overhead, coming into land some distance away. They run off to investigate further.
At a large industrial estate elsewhere on the planet Mr Halpen arrives to speak with two of his employees, a scientist named Dr Ryder and a woman named Solana. They explain that three people have died in the complex in the past few months and show him security footage of Mr Bartle being killed. They theorise that the Ood must be to blame for the other deaths and attribute this behaviour to a condition known as ‘red eye’. Halpen asks where the Ood that killed Bartle has gone, and they explain it is being tracked down. Halpen listens as his assistants explain the change in Ood behaviour, whilst taking time to drink a dose of hair tonic brought to him by his assistant Ood Sigma.
Meanwhile, the Doctor and Donna continue to explore. The Doctor stops suddenly, claiming he can hear a song being sung inside his head. He tries to follow it to a source and eventually discovers an Ood lying down, half buried by the snow. He examines the creature and Donna tries to comfort it. It tells them ‘the circle must be broken’, and it’s eyes glow red. It snarls at them but moments later it is dead, leaving the Doctor and Donna alone.
The Doctor explains that he has met the creatures before, and recalls how they were possessed by an evil force that claimed to be the devil. He claims the creatures are telepathic, hence the song he heard in his head. Donna claims she heard nothing and together with the Doctor she leaves the creature to be buried by the falling snow. As they walk on the Time Lord explains the creatures are usually benign, but something has gone wrong.
Some time later they arrive at the industrial complex, where Solana is greeting a group of other visitors who have come to visit the company and discuss business deals regarding the Ood. The two travellers claim to be representing a company and Solana gives them information packs before welcoming them inside. As they go an alarm sounds.
In his office Mr Halpen, after drinking more of his hair tonic, contacts his security officer, Commander Kess, and asks what is going on. Kess tells him an Ood with ‘red eye’ is now on the loose within the complex. Whilst Solana conducts a business lecture about the Ood and their functions as servants, the security team track down the rogue Ood, which not only has red eye but also appears to be rabid with anger. Speaking over the intercom Halpen orders them to take the creature away, then resumes looking at himself in the mirror, trying to decide if his hair is growing back yet.
Solana shows her guests further information about the Ood, who are being sold as servants across the Second Human Empire. The Doctor identifies the time zone as being the 42nd Century, and Donna confesses she never expected mankind to live so long – what with continual signs of impending doom such as global warming and the bees going missing occurring in her own time.
She asks one of the Ood servants if there are any Ood who are free to roam, but it insists they are bred to serve. Like the Ood they found dying in the snow, it claims ‘the circle must be broken’. Not content with the official tour of the complex, the Doctor decides to take a look around for himself, and Donna is soon alongside.
In one of the other buildings Halpen and Dr Ryder examine the rabid Ood captured by Commander Kess. Halpen is concerned that other Ood may start to show similar signs of illnesses, which could put the company out of business. Ryder theorises the answer may lie in Warehouse 15, but Halpen insists what is contained there is not to blame; it has been dormant for two centuries. He calls for more hair tonic and his personal Ood servant complies. He decides to visit Warehouse 15 anyway and tells the nearby guards to shoot the captured Ood so a post mortem can be carried out.
The Doctor and Donna look on as crowds of Ood are paraded around the complex, those who fall behind being beaten like slaves. They then see Halpen walking around and, assuming him to be in charge of the company, decide to stay out his way. They leave too soon to see Halpen enter Warehouse 15. He emerges with Dr Ryder on a balcony overlooking something far below. Nothing appears to be wrong, but Ryder conducts some checks anyway. Halpen continues to look over the balcony before telling Ood Sigma: “say hello to daddy”. He is interrupted when Solana calls him over the intercom to tell him two visitors are missing. He tells her to send out a search party but not to sound any alarms en case the remaining guests grow disgruntled and take their business elsewhere. He takes another glass of hair tonic from Ood Sigma and tips it over the edge of the balcony and onto the creature below, which screeches in discomfort. Halpen downs another glass, and then leaves.
Outside, the Doctor and Donna make their way toward another warehouse, where they find hundreds of large metallic containers being shipped off to the rest of the empire. They open one of the strongboxes and discover each one is full of Ood, all crammed together. Although the doors are unlocked they do not try to escape, because they have nowhere to go. The Doctor asks if the Ood know of ‘the circle’ mentioned by the other Ood they have encountered and they claim in unison that it ‘must be broken’. When asked why, they claim it is so they ‘can sing’.
Halpen receives a call from Commander Kess claiming the security team have located the missing visitors in the cargo hold. They set off an alarm despite instructions not to and the Doctor and Donna run off, but are separated in their attempts to find an exit. Donna is captured by the guards and thrown into a container with the Ood, which is then locked. The Doctor is left outside but is chased around the hold by Commander Kess, who operates a large claw-like object otherwise used for transporting the containers. The large metal grabber pursues the Time Lord around the myriad of metal Ood transportation caskets, occasionally making an attempt to grab hold of him but continually proving unsuccessful.
Inside the locked container the Ood awaken, their eyes glowing red. They advance on Donna, who calls out to the Doctor for help. Meanwhile, Solana, who explains that Mr Halpen wants the two strangers kept alive, has stopped Commander Kess in his murderous pursuit of the Doctor. The guards collect the Time Lord, who has been knocked to the ground by the claw, and retrieve Donna from the container, ready to take them away. However, the security team are left with other concerns when the rogue Ood begin leaving their container, and begin murdering anyone who gets in their way.
Solana flees the scene with the Doctor and Donna and when requested tells them where they can find Dr Ryder, the man who will be able to help them understand what is going on with the Ood. The Doctor tells her to come with them but instead she alerts the guards to their presence. Once they have run off, she calls Mr Halpen to let him know where they are going – Ood Conversion.
Halpen calls Commander Kess, who explains that the Ood with red eye have been contained in a cage, but more of them are going rabid. Halpen downs more of his hair tonic and then orders that the Ood be culled before they can escape or infect any of the others. Kess agrees, and goes to fetch some gas canisters.
Meanwhile, the Doctor and Donna make their way to the Ood Conversion section of the complex, where they discover a group of Ood who are yet to be processed. The Doctor again complains of hearing singing in his head, and upon her request manipulates Donna’s mind so that she is able to hear the song as well. However, after a few moments she breaks down in tears and asks him to stop it, unable to bear the song of the Ood’s captivity.
Halpen arrives outside the chamber but is unable to enter for the Doctor has fused the lock with the Sonic Screwdriver. Inside, the Time Lord examines the unprocessed Ood more closely and discovers that before they are conditioned, the creatures carry a secondary brain in their hands rather than a communications orb. Removing the brain removes their emotions, without it the creatures become unthinking slaves.
Donna is distraught by the things she has seen and confesses she wants to go home; life with the Doctor is not what she imagined. Halpen arrives with his guards and has the two intruders taken to his office, assuming them to be peace protesters working in favour of the Ood. He claims the creatures did not resist being captured when humans first explored the planet, but Donna insists they did not do so because of their vulnerability – they must trust in whomever they can.
Halpen checks that the gas containers are in place and Commander Kess confirms the toxic fumes will be released in a few minutes. However, the Ood begin to fight back. Those contained in the makeshift gas chamber join hands in a circle, and use their telepathic ability to transmit the red eye condition to all the Ood on the planet. They begin turning on every human in the base, including the business visitors welcomed by Solana.
The Ood manage to escape the gas chamber and lock Commander Kess inside. Seconds later the gas canisters activate and he is drowned in toxic fumes. Halpen, meanwhile, decides to leave the base before he can be killed or blamed for what has happened. He decides to leave the Doctor and Donna tied up, hoping the Ood will track them down and kill them when they spread through the base. Before he can go, however, the Doctor takes the chance to ask if he is correct in thinking there is a third element of the Ood brain, alongside the brain in their head and the one replaced by the communications orb. It is this element that prevents them from going mad when the second element is removed. Halpen reveals the Time Lord is correct, and congratulates him on his intellect before fleeing with Dr Ryder.
He arrives outside and prepares to make his escape, but not before giving Ood Sigma leave to join his own people as they revolt. Inside the complex the Doctor struggles to undo the handcuffs binding himself and Donna. The Ood enter and advance on them, ready to kill, but stop when the Ood from the conversion chamber telepathically command them to stop, claiming the Doctor and Donna are friends. Their red eye disappears and they free the captive travellers.
The two friends run outside whilst the rest of the Ood continue their rebellion, battling to overcome the firepower of the human guards. With the help of Ood Sigma the travellers follow Halpen and Dr Ryder to Warehouse 15, where the two men are planning to destroy the creature stored inside, the third element of the Ood brain. Halpen claims that if the creature is destroyed, so are the Ood. They begin placing detonation devices around the warehouse parameter when the Doctor arrives. He glances over the edge of the balcony and discovers the creature is in fact a giant brain, one that telepathically links all of the Ood and allows them to share their song.
However, the brain is surrounded by a circle of pylons, which create a static barrier capable of preventing the Ood from communicating telepathically. This is the circle that must be broken so that the Ood can be free. Having planted all of the detonation devices, Halpen arrives ready to leave. Before he can, Dr Ryder reveals he is a secret agent for Friends of the Ood, a peace protest group opposed to the treatment of Ood across the empire. He infiltrated the complex and lowered the power of the barrier surrounding the brain so that the Ood could begin to communicate again and gain the chance to break free. Betrayed, Halpen turns on him and throws him over the barrier. He lands on the brain and is absorbed into it.
Halpen, who is beginning to show visible signs of illness, then pulls a gun on the Doctor and Donna. However, but before he can pull the trigger Ood Sigma stands in front of the travellers and offers his master a drink of hair tonic. Halpen declines the offer but Sigma is insistent. Halpen thinks for a second and fears that his servant has poisoned him. Sigma claims this is not the case, as the Ood are not permitted to kill. Halpen is, however, correct in guessing the liquid he has been drinking is not hair tonic, it is Ood DNA. The Doctor realises what is going on and guesses correctly that Halpen can now hear the Ood’s song. He watches with Donna as Halpen reaches to the top of his scalp and, with a mighty effort, begins peeling away the skin on his head, revealing the skull of an Ood beneath. His mouth opens and he heaves - causing tentacles to fall out – he has become one of the creatures he enslaved. Bemused by this, Donna comments that having spent time with the Doctor she has no idea what is right and what is wrong any more. He claims this is for the best.
The Doctor suddenly recalls the detonation devices attached to the building and hurriedly de-actives them, before doing the same with the barrier surrounding the giant brain. With the circle broken at last, the Ood are freed. The conflict outside stops and the liberated Ood hold their hands to the sky, singing a song of freedom – one that even Donna can hear.
Some time later the Doctor and Donna stand beside the TARDIS, ready to leave. The Doctor explains that the new song will have been heard around the entire empire, and as a result the Ood will all be brought home. Ood Sigma offers them a place in the song as well, but the Doctor declines, claiming he has a song of his own. Ood Sigma agrees but comments:
“I think your song must end soon.”
The Doctor asks what he means and Sigma tells him that every song must end eventually. The Time Lord considers this but then turns to Donna instead and asks her if she still wishes to leave his company. She claims that she has changed her mine again, and together they depart in their TARDIS, aware that their names will not go with them, they shall remain to become part of the song.