Inside the TARDIS the Doctor decides to extend his initial invitation to Martha of only taking her on a single trip; deciding to take her once into the past and once into the future before finally taking her home. He decides to take her to a different planet but disregards the idea of visiting his home-world. Martha continues to ask what it is like and eventually he tells her of the burnt orange skies above the glass-domed Citadel, beside the red mountains capped with snow beneath the twin suns.
He tells his young friend there would be no fun for him in returning home and instead opts to take her to New Earth in the year 5 billion and 53, to the city of New New York. They step outside and as the Doctor removes the arrow lodged in the ship door from their previous trip, Martha is less than impressed by the torrent of rain that awaits them.
They move through a dark alleyway searching for cover as elsewhere in the city the Face of Boe has sensed their arrival. He tells the nurse attending him to find them before it is too late, and Novice Hame gets to her feet in obedience, arming a gun.
Back outside Martha fails to see what is so new about the planet but after the Doctor finds an information screen and activates it with the Sonic Screwdriver, he shows her a view of the city, a collection of shining buildings ringed with flying cars surrounded by vast stretches of water, the place he stayed on his last visit.
Martha comments that he has brought her to the slums before asking if this was a place he brought Rose. He admits that it was and her reaction is not a happy one.
“Even heard the word ‘rebound’?”
Soon the rain stops and as they move into another alleyway three proprietors concealed in tiny outlets built into the walls detect their arrival. They open up shop and all talk at once, trying to sell them “happy” and “anger” amongst other emotions. Martha is unimpressed by the similarities to drug dealing and she looks on as a young woman approaches one of the vendors.
She explains that her parents have gone onto the motorway and she wants to buy a “forget” patch. She hands over the money but before she can apply the patch the Doctor cuts in. He fails to understand her problem but she insists that nobody returns from the motorway, everyone goes there in the end.
She attaches the patch to her neck and saunters off, leaving Martha to look on in disgust. Suddenly, through the fog of the alley two people; a young man and a young woman lunge forward and grab her. Holding a gun to her head and apologising profusely they back off, the Doctor demanding that they let his friend go.
They make their way to a doorway and slam it shut. The Doctor struggles to open it again but soon he is on their tails. The young couple arrive at a vehicle some distance away and to calm Martha down they apply a “sleep” patch to her neck. She slumps down and they bundle her into the black, block-like shuttle. They strap themselves in and soon the car rises into the air zooming off, leaving the Doctor alone on a nearby balcony.
The young couple check Martha is OK before reporting into an onboard computer that they have three passengers, and would like access to the Fast Lane. A voice grants them entry and they smile.
Returning to the alleyway the Doctor demands that the vendors tell him where the duo has taken his friend, and they explain that they will have gone to the motorway. He asks why they needed three people and one of them explains that if a vehicle carries three adults then they have access to the Fast Lane. He vows to get her back and then tells them to pack up and leave, once he has returned the street will close.
Back inside the car Martha awakens and removes the patch on her neck. Hazily she reaches for her kidnappers’ gun lying nearby. Aiming it at them she orders them to take her back but they reveal the gun is a fake. The young woman introduces herself as Cheen, and her boyfriend as Milo. She explains why they abducted her and then tells Martha that once they have arrived at their destination she will be free to go.
Cheen swears she is telling the truth and reveals an “honesty” patch on her neck. Milo reveals that they are on their way to Brooklyn to start a new life; Cheen is pregnant. Martha scolds her for wearing the patch in her condition and asks how long the journey will take. They explain the route is only ten miles long; they should be there in about six years.
The Doctor finds an access point to the motorway and when he goes through he finds himself in a giant metallic tunnel, with thousands of identical cars all stuck in jams sounding their horns. He begins to choke on the fumes given off by the vehicles until the car hovering next to him opens up, and a masked passenger calls him inside.
They close the door and the stranger, a male Catperson with an Irish accent, talks about the dangers of overexposure to the atmosphere outside. He takes his seat by the steering mechanism and as a gap opens ahead of them he lurches the vehicle forward. Commenting on a good days progress of twenty feet he introduces himself as Thomas Brannigan, and the human woman sitting beside him as his wife Valerie. The Doctor also introduces himself before finding the rest of the family, a pack of kittens, sitting in the back of the van.
Brannigan tells him they are two months old; children of the motorway, and then explains that he and his wife have been on the road for twelve years, having only covered five miles. The Doctor is astounded and then realises he must find Martha. Brannigan points out they have moved past the lay-by, and they won’t arrive at the next one for six months.
Back in one of the other cars Martha marvels at the thousands of similar vehicles outside, as Milo explains that the Fast Lane is at the bottom of the entire motorway. Cheen begins to talk quite fondly about the idea of living inside the car, but Martha is still not impressed.
The Doctor tries to call the police but is put on hold. Valerie explains that the entire motorway is enclosed so outside calls are impossible; the only contact is with the cars outside who are added to each driver’s “friends list”. Brannigan calls up the Cassini sisters, an elderly couple who were amongst the first to join the motorway over twenty years ago. One of them manages to locate the car in which Martha is being held and gives him the registration number. However the Time Lord still has no way of reaching them.
Now ten layers of cars away from the Fast Lane Martha is confused by a roaring sound coming from below. Milo states it is the air vents but Cheen relates a story that claims something lives at the bottom of the motorway, feeding off of lost drivers. Milo again states his theory but Martha is sceptical; judging by the filthy air outside the air vents are no longer working; yet the noise still continues.
The Doctor asks Brannigan to take him to the Fast Lane but both he and Valerie refuse. He calls the Cassini sisters again and asks if they have ever seen a police car or an ambulance on the roads, but they admit they never have. He theorises that there is no one out there, the cars are forgotten and continue to drive round and round, with no end in sight.
Valerie silences him before Sally Calypso’s voice cackles into earshot; announcing the daily contemplation. Brannigan claims the Doctor knows nothing of the drivers on the roads, and the Time Lord looks on as across the motorway a hymn can be heard.
“On a hill far away stood an old rugged cross,
The emblem of suffering and shame;
And I love that old cross where the dearest and best
For a world of lost sinners was slain.”
All of the drivers solemnly sing along, their eyes welling up. Even Martha, through her gathering tears, manages to mumble a few lines.
“So I’ll cherish the old rugged cross,
Till my trophies at last I lay down;
I will cling to the old rugged cross,
And exchange it some day for a crown.”
Milo, Cheen and Martha receive access to the Fast Lane, and dive below the traffic to an empty road shrouded in the sickly mist.
The Doctor decides to find Martha on his own. He opens a hatch in the bottom of the car and throws Valerie his coat. Brannigan laments that Martha must mean a lot to him but he confesses he hardly knows her; and yet he lied to her. In an instant he jumps through the gap and onto another car roof.
Beginning to choke on the fumes again he uses the Sonic Screwdriver to open up the roof of the car. Astounding the driver inside he claims to be from the Motorway Foot Patrol, and as quick as he arrived he departs, continuing through the floor.
Soon he begins his descent, slowly clambering through the layers of cars, surprising the passengers in each vehicle. Unable to cope with the fumes anymore he borrows a handkerchief from one of the drivers, continuing his journey with it tied over his mouth.
Milo and Cheen are devastated to find all exits to Brooklyn closed and the noises outside gradually growing louder. Suddenly the vehicle begins to shake and Milo insists that nothing could survive outside in the fumes. Suddenly they receive a call from another vehicle on the Fast Lane, which is being attacked by something outside.
The driver tells them that all of the exits are always closed, and warns them they must leave, but Milo claims they don’t have access to drive upwards. The woman’s passengers scream out in terror and soon the line is cut off, and Martha tells the couple to rush forward as fast as possible.
Elsewhere the Doctor arrives at the final layer before the fast lane. The driver, a well-dressed businessman, gives him some water before the Time Lord opens up the hatch on the floor. Seeing nothing below but the fumes dotted with distant lights he listens to the loud grumbling and sets about using the onboard computer to create some kind of a breeze.
Back in Brannigan and Valerie’s car the roof hatch falls away in a shadow of sparks. Suddenly Novice Hame appears with her gun, demanding to know where the Doctor is.
The Doctor manages to create a draft beneath the car and as the clouds thin out he realises what is waiting below.
Milo drives the car flat out but the roaring continues and the vehicle keeps shaking, whilst outside giant claws snap at the tiny shuttle. Calls to the police continue not to work and Martha comes up with an idea; switch everything off to see if it is the car itself that provokes the creatures. Milo does so and suddenly all falls silent and still. Cheen congratulates her but a bigger problem faces them; the air con has stopped and in eight minutes the air will run out unless they start the engine again.
The Doctor explains to his erstwhile companion that the Macra were the scourge of the galaxy, feeding off of unrefined gas. They built a small empire and used humans for salves but now, billions of years later; they have devolved into savage beasts.
Suddenly the roof above the two men gives way and Novice Hame clambers inside. The Doctor initially fails to recall who she is but when he does he embraces her; before realising that the last time he met her she was helping to breed humans for experimentation.
She claims her gun was only for protection before explaining that she has sought forgiveness and if he follows her she may retrieve it. She grabs hold of his arm and after activating a transporter they vanish from sight.
They arrive in a tumbledown building somewhere above the alleyways of the slums. The Doctor demands to see the Senate of New New York to complain about the people trapped on the motorway, but Hame points out he is already there. She turns up the lights to reveal the grand building surrounding them; lined with the skeletons of the dead Senators.
Hame explains they and the rest of the city died twenty-four years ago after the development of a new mood drug called “Bliss”. The entire city became addicted and eventually a virus mutated within it, wiping out the entire population and eventually itself. The undercity and the motorway were sealed off but there was not enough power to open it up again once the danger had passed.
The Doctor asks who helped her do this and suddenly he hears a voice calling his name. He runs across the room to find the Face of Boe waiting for him. Hame explains she was made his Nurse as penance for her sins and when the virus came he protected her. Now wired into the mainframe Boe has been giving his life force to protect the undercity, unable to call for help due to the quarantine the planet was placed under.
The Face of Boe pleads for the Doctor to help, whilst deep beneath them Milo, Martha ad Cheen sit in silence. Martha laments the Doctor coming to rescue her but begins to realise how little he has told her about himself, and how her parents would never know if she died on an alien world.
Cheen notes that their only hope is a complete stranger but Martha insists he is more than any other man. Just as the drivers have their faith and their hymns, she has the Doctor. Milo starts the engines again and as the noise outside rears up once more they accelerate through the minefield of crab-like creatures outside.
In the Senate the Doctor discovers that Martha’s car still registers on the computer systems and sets about restoring the power. Hame helps him with the burnt out computers but the Doctor’s confidence is misplaced. He tries to activate the machines but has made an error; the transformers need to be fixed first. He continues his work as the Face of Bow struggles to speak.
“Doctor…I give you my last…”
Suddenly the power begins to return and as Hame tends to her patient the motorway begins to shake. High above the cars the roof slowly opens. Sections of the ceiling fold away and daylight floods inside. The drivers look on in amazement as the Doctor’s face appears on the computer screens, telling everyone to drive up into the skies and clear the Fast Lane.
Milo manages to fly his vehicle out of the Macra’s reach and as Martha jumps for joy the whole motorway rejoices as they fly towards their freedom. The Doctor continues to speak to Brannigan and reminds him to return his coat, before telling Martha to come to the Senate. However as he speaks Hame calls to him. He turns around and watches as the glass container in which the Face of Boe resides begins to crack.
Some time later Martha arrives, and finds her friend crouched over the dying body of Boe; the glass container smashed. The Doctor recalls the legends that surround the Face of Boe and Hame claims that at the end of his life, her patient will impart his last secret to a traveller.
Boe begins to speak, recalling the many things he has seen in his long life. He is the last of his kind just as the Doctor is the last of his. The Time Lord asks him not to go but he is too late, and Boe speaks his last.
“Know this Time Lord…You are not alone.”
The Doctor’s brow becomes furrowed as Boe breathes his last and Hame begins to cry. He gets to his feet and walks over to Martha, putting an arm around her.
Having retrieved his coat the Time Lord returns with his companion to the undercity, where the vending stations have closed down. Martha asks what Boe meant and ponders if he meant that the Doctor is not alone as long as he has her. Her friend claims that that was not the secret, but then dismisses the importance of the message altogether and instead moves to return to the TARDIS.
Martha however is not impressed so instead of following pulls up an abandoned chair and sits down in the middle of the alleyway. She demands that the Doctor explain why he is the last of his kind and as the city begins to sing again the Doctor talks. He explains that he lied to her, that way he could pretend his race were still alive. He is not just a Time Lord. He is the last of the Time Lords. The Face of Boe was wrong; there is no one else.
He pulls up a chair and sits before her, explaining that his people were destroyed in the last great Time War, fighting for the sake of all creation against a race known as the Daleks.”
“They’re all gone now. My family, my friends even that sky. Oh you should have seen it that old planet. The second sun would rise in the south and the mountains would shine. The leaves on the trees were silver. When they caught the light every morning it looked like a forest on fire. When the autumn came breeze would blow through the branches and sound like a song…”
And as he talks the singing far above continues, the setting sun silhouetting the tall buildings of New New York; the skyline now bustling with life.