This week's episode is from the pen of chief writer, Russell T Davies, and it is, frankly, bonkers - but still good fun. The Doctor has relented a little on his decision to only take Martha on one trip in the TARDIS, and decides she can have one trip to the future and one to the past. With that in mind he takes them off to New Earth, although Martha is rather keen to see his home planet. He says he doesn't want to go home (liar!) although he does tell her a little bit about how it looked (whilst omitting to mention that Gallifrey is no longer there).
They arrive in New New York (giving David the chance to rattle off his "New, new, new (15 times) York" line again). It's raining which fails to impress Martha. The Doctor yanks the arrow (shot at him by one of the 16th century soldiers of the Queen in last week's episode) from the TARDIS door and discards it, before chiding her for moaning about the rain and insisting that seeing the city from down below is more interesting. Martha complains it looks like Earth on a Wednesday afternoon as the Doctor checks out a computer terminal to see exactly where they are. When he mentions that "we" saw the view he shows Martha on the screen "last time" (of the hospital from "New Earth"), Martha ask if he came here with Rose. He says yes and she comments on the fact he's taking Martha to the same planets to which he took Rose, and makes a snide comment about "rebound" (which I thought was rather unnecessary - given the Doctor's not in a relationship of any sort with Martha. He freely admits later on that he barely knows her because he's been too busy showing off to her - and that he'd lied to her).
The two of them walk down an alley and find themselves accosted by three dealers of mood patches. Then a pale young woman turns up, wanting some "Forget" as her parents have gone on the motorway, and she believes (rightly) that they'll never return. Before the Doctor can sort out why that might be, she's attached the patch to her neck and forgotten about her parents. She wanders off and then Martha is abducted at gunpoint by a man and a woman who are babbling about needing a third, whilst madly apologising to both Martha and the Doctor. The Doctor tries to persuade them to let Martha go, offering to help, but they're not interested. They bundle Martha away to their waiting "car", give her some "Sleep" and then drive off to the motorway, requesting access to the Fast Lane as they now have three adult passengers on board.
It turns out the motorway in the undercity of New New York is entirely enclosed and suffers from a traffic jam that makes even the M25 look like an easy Sunday drive. Some citizens have been stuck on it for over 20 years (that's definitely one of the more bonkers bits of the plot). The Doctor tries to go after Martha and is picked up as a hitch-hiker by Thomas Kincade Brannigan, a cat man (played magnificently by Ardal O'Harlon), and his wife (a regular human), who, along with their children (a group of impossibly cute kittens, whom even the non-cat-person Doctor can't resist petting). They explain the problems with the motorway to the Doctor, who contacts the New New York police, only to be put on hold. He tries to persuade Brannigan to take him down to the Fast Lane, so that he can go after Martha, since they now have three adult passengers, but Brannigan and Valerie both refuse to endanger their children. After a brief "Contemplation moment" (in which the traffic news system plays "The Old Rugged Cross"), the Doctor decides to take things into his own hands, and lets himself out of the bottom of Brannigan's car (after leaving his coat, given to him by Janis Joplin, with Brannigan), then drops in through the roof hatch of the car below. (Two of the best lines in the show are Valerie's response to the Doctor's action: "He's completely insane!" and Brannigan's response "That and a bit magnificent!")
Eventually the Doctor reaches the last car above the Fast Lane and does some jiggery-pokery with the wires in the car to clear away the exhaust fumes below so he can discover just what's down there that has eyes and makes weird noises. Turns out it's the Macra - a bunch of super-sized crabs that like to feed on gases, the dirtier, the better. They used to be the scourge of the galaxy but now they're just lurking about in the enclosed motorway (don't ask how they got down there), living off the gas and attacking cars in the Fast Lane (presumably for sport or out of a general antipathy towards humans and human/hybrids). Just as the Doctor's discussing this with the businessman car owner whose car he dropped into, someone else drops in (prompting the Doctor to claim he's invented a new sport) - the someone being a cat woman, formerly Novice Hame of the New Earth hospital where Cassandra (the "bitchy trampoline") managed to transplant her brain into both Rose and the Doctor - prompting some truly magnificent campness from David Tennant that was just the right side of outrageously silly. Hame's come to fetch the Doctor to meet an old friend - the Face of Boe (who had contacted the Doctor via his psychic paper, thus prompting him to visit New Earth with Rose in the first place).
Hame reveals that the Face of Boe is finally dying (he was supposedly dying in "New Earth", hence his desire to see the Doctor) and that the Senate of New New York are all dead after being killed by a virus that was part of the new "Bliss" patches. The people who are in the motorway are the only survivors. The Face of Boe kept them alive by wiring himself into the system but there isn't enough power to let them out of the motorway. Fortunately the Doctor is able to do some more jiggery-pokery and with some power from the Face of Boe, he's able to unlock the motorway and get everyone out. He tells Martha's kidnappers to bring her to the Senate building and introduces her to Boe, who's really dying now. Before he goes, however, he reveals his big secret - that the Doctor is not alone, although he is the last of his kind. Which leaves the Doctor confused and somewhat angry. However, he reclaims his coat from Brannigan and heads back to the TARDIS. Martha, however, wants some answers from him, and she picks up an old chair and sits down, refusing to go another step until the Doctor talks to her. They hear the people of the city singing "Abide With Me" and he finds another chair and sits down. He then reveals that he'd lied to her, and explains that Gallifrey was destroyed in the last great Time War, against the Daleks, and he begins to describe it to her, as the camera pans upwards away from them, and the hymn continues...