After a brief pre-titles sequence in which something nasty appears to be happening to a couple in a car, we join the Doctor and Martha in the Tardis. The Doctor starts off by saying that having given Martha that “one trip” he promised her, it’s time to take her home. But then he decides to stretch it a bit and make it one trip into the past and one to the future, and suggests visiting a different planet. Martha asks about the Doctor’s planet, and rather than tell her the truth about its destruction, he spins a fantastical description of his home. Martha naturally wants to go there, but instead, the Tardis arrives on New Earth. Things seem to have changed a bit since our last visit. In the undercity, there’s an air of decay, with a few tatty stalls selling “moods” – patches that give the wearer the feeling they want. Or that can remove painful memories. The travellers are just getting their bearings when Martha is dragged off by a very apologetic young couple with a quite large gun.
Up in the city, we once again see the ancient Face of Boe, who instructs one of those lovely Cat Nuns to find the Doctor “before it is too late”.
Back in the undercity, the Doctor is angry. Angry that Martha has been kidnapped, but mostly angry with himself for getting her into trouble. So, off he goes. He finds the access door to the motorway, which turns out to be full of identical flying vehicles which reminded me of old Commer vans, though that may be senility setting in. The motorway is fully enclosed, and there seems to be a slight pollution problem – so bad that the Doctor is on the verge of choking when he’s picked up by Brannigan, a cat person and his human wife Val. And their kids, some outrageously cute little kittens.
We soon learn why Martha was kidnapped. In order for the automatic system to allow a car into the fast lane, right at the bottom level of the motorway, it has to have at least three people on board. We also learn that the traffic is moving very slowly. Very very slowly. Hardly moving at all, in fact. I think RTD  might have had a bad experience on the M25 which led him to write this story.
Martha and her apologetic kidnappers reach the fast lane, where they find that all is not quite as it should be. Are the strange noises really something nasty living at the lowest level of the city, or is it just the air vents? Well, given the air quality, and air vents probably stopped venting a long time ago, so it’s probably the something nasty.
The Doctor adopts a slightly unorthodox method for getting down towards the fast lane. He drops from the hatch in the bottom of Brannigan’s car to the roof of one below it, then opens the roof hatch of that car and drops into it. And so on, passing through a lovely selection of car interiors with a nicely eccentric mix of characters.
Finally, he’s just above the fast lane. Looking down, he sees what Martha and the kidnappers have already seen – there really is something nasty down there. Giant crabs to you and me, but the Doctor recognises them as Macra, creatures he last encountered many years ago.
Just as he’s about to do something drastic, he’s joined in the car by that Cat we saw earlier. She turns out to be a slightly older Novice Hame, whom the Doctor met on his last visit. Over his protests, she teleports him up into the city to join the Face of Boe.
The City is dead. Years before, a rogue virus developed from a popular mood, and killed everyone. Boe used his power to protect Novice Hame, and for all the intervening years, the motorway was sealed to protect the people there. After the usual fun and games, the Doctor manages to open the roof above the motorway, and instructs everyone to drive up into the light, clearing space for Martha to escape from the claws of the Macra.
But Boe gave the last of his strength in the process, and finally, after an immensely long life, he is dying. And at last, he reveals his final secret to the Doctor:
I am the last of my kind, as you are the last of yours. But know this, Time Lord:
You are not alone!
Which is nicely enigmatic, and as long-time viewers will know, not as contradictory as it might seem. In The Keeper of Traken, the Master stole the body of Tremas, and by the time of the 1996 TV movie, he was able to transform into a snake-like form that stole another body. So, if the Master is still around, he isn’t strictly a Time Lord.
After all that, before getting into the Tardis, the Doctor apologises to Martha for not telling her the truth about his world. He tells her about the Time War, and the Daleks.
Which sets us up nicely for the next episode: the first of a two-parter set in 1930s New York featuring the Daleks.
This was a “harmless fun” episode, really. It showed the Doctor realising that he actually does care about Martha, and leads in nicely to the Dalek story, but its fundamental purpose was Boe’s secret. Which was big enough to make it the whole point.
 You may well ask how cat-shaped kittens will grow into the more humanoid cat person form, or cat-human hybrid, but that would just be picky.
 Russell T Davies, do keep up
 We were promised the return of an old enemy from the 1960s. This would be it. A nicely obscure and moderately silly choice.
 And there’s approximately zero doubt about that