The Tenth Doctor takes Martha to an alleyway in the city-state of New New York on New Earth where street vendors are selling addictive mood drugs in the form of sticky patches to help people deal with their emotions. A young couple named Milo and Cheen kidnap Martha. 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. 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. One of the motorists, Brannigan, helps the Doctor locate the vehicle containing Martha, which is heading towards the fast lane. The Doctor tries to call the police but gets put on hold. Upon noting that the television broadcasts in the vans are looped, the Doctor deduces that there is no way out of the Motorway and tries to reach Martha himself by breaking into and out of vans on the way to the fast lane.
Martha, Milo, and Cheen drive into the fast lane and see crab-like Macra, 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 to hide in the fog. Novice Hame finds the Doctor and teleports him against his will to the Senate of New New York. Hame explains that a virus mutated in the drug “Bliss” and wiped out the entire surface population. Eventually the virus also perished. Those in the Motorway were spared by being sealed in and the planet was quarantined. The Face of Boe wired himself to the system to keep the Motorway operational but could not unseal it himself. The Doctor works with Hame to unseal the Motorway. The Face of Boe gives the last of his energy to the system, allowing the ceiling of the Motorway to open and freeing the motorists. When Martha arrives at the Senate, the Face of Boe, close to death, imparts his final message to the Doctor: “You are not alone.” The Doctor tells Martha that the Face of Boe is wrong; he is the last Time Lord and his planet was destroyed in the Time War.