Gridlock

The Doctor takes Martha with his TARDIS to the year five billion and fifty three in New New York on the planet New Earth, a place to which he had previously taken Rose Tyler. They end up in an alleyway where street traders are selling mood patches to help people deal with "The Motorway", but as they are investigating this, Martha is kidnapped at gun point and drugged by a young couple, Milo and Cheen. Once in their vehicle, they explain that Cheen is pregnant and that they needed three adult passengers to qualify for the high-occupancy vehicle lane, the "Fast Lane", and that they will drop Martha off when they reach their destination 10 miles (16 km) away in six years. Cheen also mentions rumours about something living down in the fast lane, saying that cars that go down there vanish, never to be seen again.
The Doctor chases after Martha's kidnappers and arrives at the (entirely enclosed) Motorway to find thousands of hover vans stuck in a tri-dimensional gridlock. The fumes in the air begin to affect the Doctor, but he is invited into the hover van of the cat person Thomas Kincade Brannigan and his human wife Valerie, who have a litter of kittens. The Doctor learns that the couple, along with all the other traffic on the road, has only travelled a short distance in several years, their vans having become their homes. Brannigan helps the Doctor to locate the vehicle containing his companion, heading towards the Fast Lane. As the Doctor tries to call the Police but gets put on hold and Brannigan refuses to put his family in danger, the Doctor decides to try and reach Martha himself and begins to jump down from lane to lane, breaking in and out of vans with his sonic screwdriver. Shortly after the Doctor leaves Brannigan's van, Novice Hame, who has been sent by the Face of Boe to locate the Doctor, enters Brannigan's van, chasing after him.
Martha, Milo, and Cheen drive into the Fast Lane, and begin to hear strange noises all around, and 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, monstrous crab-like alien beings that once ruled an empire but now have devolved into feral, non-sentient beasts, who attempt to capture and eat those that fly in the Fast Lane. As the Doctor watches, Milo's van is nearly caught by one Macra's huge 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 away forcefully to the Senate where the Face of Boe is waiting. Hame explains that a "Bliss" drug created by the pharmacists mutated into a virus that became airborne and wiped out the entire surface population, and only those in the Motorway were spared due to her actions to seal it off, keeping the travellers in constant traffic to avoid telling them the real problem. The Face of Boe has 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 but there is not enough energy left. The Face of Boe sacrifices his life energy to the system, allowing the ceiling of the Motorway to open, and the Doctor broadcasts a call for all drivers to escape that way and for Martha to come find him.
When Martha does so, the Doctor and Novice Hame are tending over the Face of Boe who is finally close to death; the glass container that encased it had shattered. The Doctor tries to encourage the Face of Boe to live, as both the Doctor and the Face of Boe are the last of their respective kinds, but with its dying breath it whispers the secret it promised to tell the Doctor before: "You are not alone". As the Doctor and Martha leave Hame to mourn, the Doctor does not try to postulate the meaning of those words, but explains to Martha that he is indeed the last Time Lord, and tells her all about Gallifrey as they return to the TARDIS.