Utopia, by Russell T Davies, is an episode from later in season 29 than Gridlock. It starred David Tennant as the Doctor and Freema Agyeman as Martha. It guest-starred John Barrowman as Captain Jack, and has Derek Jacobi in it as well!
It starts with the TARDIS appearing in Cardiff and Captain Jack runs up to it and holds on when it vanishes again. He's still holding on when it travels in time. I think they should have shown him still holding on all the way through the titles but they didn't do that.
On another planet there are scary monster-mannys with sharp teeth, and they are chasing another manny like a cat chasing a bunny: for noms. Then we see Derek Jacobi! He is Professor Yana. He hears drums in his head and has an assistant called Chan Tho, she is an insect-manny who talks in a funny way.
Captain Jack is dead but comes back to life because he is Captain Scarlet now. This is the fault of rubbish Torchwood because it took the very nice Captain Jack of Doctor Who season 27 and made him rubbish like the rest of Torchwood.
The Doctor and Martha meet Captain Jack when they come out of the TARDIS. They see the manny being chased and try to help him because they are goodys. They get in to a safe place where lots of other mannys are.
The chased manny finds his family so he doesn't need to be in the story any more. The Doctor finds a big rocket and then meets Professor Yana who is trying to make the rocket go. There is a monster-manny in the place pretending to be a normal manny, but the goodys don't know she is there so they are in danger.
Captain Jack has the Doctor's hand in a jar from when it was cut off and grew back. This is the fault of the rubbish Christmas Invasion because it had the Doctor's hand getting cut off when he could have just been pretending, which would have been a much cleverer thing to do.
The mannys want their rocket to go to Utopia, which is the name of another planet where they will be safe from monsters. Professor Yana made the rocket and the Doctor helps make it work. Professor Yana hears drums again when he sees the TARDIS and he tells the Doctor about it.
"No rest for the wicked."
Professor Yana also tells the Doctor that he is not a real professor; he just knows things and is clever. He is like Professor Cat in this way.
There is a red room full of radiation. It is red so we know it is dangerous. The monster-manny breaks an important thing so the radiation kills the manny in the room. Captain Jack goes in to the room because he is the only manny that will not be killed in it, and he fixes a thing that was broken to make the rocket work.
The rocket is ready to go. As it counts down Yana hears voices in his head. He has an old, broken watch. He opens the watch and gets turned in to a Time Lord by it. Martha has realised this is what Face of Boe meant when he said "You Are Not Alone" and she tells the Doctor.
This scene is very dramatic, but what happens next is even more dramatic, because although Yana was a goody he is now turned in to a baddy! He locks everyone out of the room except Chan Tho (who was there already), and then he lets the monster-mannys in to chase the Doctor, Martha and Captain Jack.
"I AM THE MASTER."
Yana is really the Master, who was a baddy in old Doctor Who and he was in Doctor Who and the Sea Devils and Frontier in Space and Castrovalva and Five Doctors and Trial of a Time Lord and maybe others that I haven't seen or don't know about yet. This is a very exciting twist.
He zaps Chan Tho with electricity but she shoots him. The Doctor gets in but the Master steals the Doctor's hand-in-a-jar and locks himself in the TARDIS. He regenerates into a silly evil version of the Doctor (who is not as good as Derek Jacobi) and steals the TARDIS.
This means the Doctor, Martha and Captain Jack can't escape from the monster-mannys who are trying to get in to nom them! It is a cliffhanger ending, how will they escape?
This episode is much better than Gridlock. It has scary monster-mannys who are very threatening, not like the silly Macra crab-monsters. But more importantly it has Derek Jacobi!
Derek Jacobi is an old manny here, like when he was Cadfael or old Claudius in I Claudius and not like when he was Hamlet or Richard the Second in Shakespeare, but he is still a very good actor. He is brilliant when he turns into the Master and becomes a baddy, it is shocking and scary - even scarier than the monster-mannys. Maybe even scarier than the Borad in Timelash.
This story builds up and up with tension and excitement until it reaches its climax in the last few minutes with a great twist and a cliffhanger. That is why this is a very good episode. So it is a shame that the episode that continues the story is a big let-down after this one...