Doctor: David Tennant (Tenth Doctor)
Companion: Freema Agyeman (Martha Jones)
Ardal O'Hanlon – Thomas Kincade Brannigan
Anna Hope – Novice Hame
Travis Oliver – Milo
Lenora Crichlow – Cheen
Jennifer Hennessy – Valerie
Bridget Turner – Alice
Georgine Anderson – May
Simon Pearsall – Whitey
Daisy Lewis – Javit
Nicholas Boulton – Businessman
Erika Macleod – Sally Calypso
Judy Norman – Ma
Graham Padden – Pa
Lucy Davenport – Pale Woman
Tom Edden – Pharmacist #1
Natasha Williams – Pharmacist #2
Gayle Telfer Stevens – Pharmacist #3
Struan Rodger – The Face of Boe
Directed by Richard Clark
Written by Russell T Davies
Script editor Simon Winstone
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 3.3
Series Series 3
Length 45 minutes
Originally broadcast 14 April 2007
The Doctor takes Martha in the TARDIS to the year five billion and fifty three and the city of New New York on the planet New Earth. They end up in an alleyway where street traders are selling mood patches to help people deal with their emotions. While the Doctor talks to a vendor, Martha is kidnapped at gun point by a young couple named Milo and Cheen. Once in their vehicle, they explain that Cheen is pregnant and that they needed three adult passengers with them to use the fast lane. They promise they will drop Martha off when they reach their destination ten miles away, estimated to take six years. Cheen also mentions rumours about something living down in the fast lane, saying that cars that go down there vanish and are never seen again.
The Doctor chases after Martha's kidnappers and arrives at the Motorway, a completely enclosed highway filled with thousands of hover vans stuck in gridlock. The fumes in the air begin to affect the Doctor and he seeks refuge in the van of Thomas Kincade Brannigan and his wife Valerie. The Doctor learns that the couple, along with all the other traffic on the road, have only travelled a short distance in several years. Brannigan helps the Doctor to locate the vehicle containing Martha, which is heading towards the fast lane. The Doctor tries to call the police but gets put on hold, and Brannigan refuses to put his family in danger so the Doctor decides to try to reach Martha himself. He begins to jump down from lane to lane, breaking into and out of vans with his sonic screwdriver. Shortly after the Doctor leaves, Novice Hame enters Brannigan's van, chasing after him.
Martha, Milo, and Cheen drive into the fast lane and begin to hear strange noises all around. A nearby driver warns them to escape into the slow lanes before being eaten by the creatures that live in the fast lane, but Milo refuses. The Doctor, finding himself in a van just above the fast lane, looks out the bottom and discovers that the floor of the Motorway is filled with Macra. The Macra are monstrous crab-like alien beings that were once intelligent but now have devolved into feral beasts, who attempt to capture and eat those that fly in the fast lane. The Doctor watches as Milo's van is nearly caught by a Macra claw, but Martha realises that the beings are attracted by the light and motion generated by the van and has Milo cut the power. This saves them from the beasts but leaves them with only a few minutes of breathable air in the van.
Novice Hame finally catches up with the Doctor and teleports him against his will to the senate building where the Face of Boe is waiting. Hame explains that a new mood drug called "Bliss" proved too addictive, and a virus mutated with the Bliss drug, became airborne and wiped out the entire surface population in a matter of minutes. Those in the Motorway were spared by being sealed in and keeping the travellers in constant traffic to avoid telling them the real problem. The Face of Boe wired himself to the system to keep the Motorway operational. The Doctor determines that whatever danger that Bliss may have caused before has long passed, and works with Hame to try to power up the systems to unseal the Motorway. The Face of Boe sacrifices his life energy to the system, allowing the ceiling of the Motorway to open. The Doctor broadcasts a call for all drivers to escape through the ceiling and gives Martha directions to come find him.
When Martha arrives, the Doctor and Novice Hame are tending over the Face of Boe. The Face of Boe is close to death and the glass container that encased it is shattered. The Doctor tries to encourage the Face of Boe to live, as they are both the last of their kinds. Knowing that it is dying, the Face of Boe imparts its final message to the Doctor: "You are not alone". The Doctor and Martha leave Hame to mourn and the Doctor tells Martha the message meant nothing. Martha knows the Doctor is lying and forces him to tell her the truth. As the undercity fills with the sound of the entire surviving population of New New York singing the hymn Abide With Me, he explains to Martha that he is indeed the last Time Lord and tells her all about Gallifrey and the Time War.