Well I can be smug, I got the plot about right :) Here is the brief story. If your going to watch this later you may want to stop reading now!
Lady Christina de Souza steals a golden challace, worth £millions, she is chased by the police and security guards, he getaway driver is caught, so escapes on a bus, the doctor also gets on this bus, scanning for something, he finds it, the bus passes through a wormhole to another planet.
After general panicking, the passengers are assigned jobs to do, to get them back through the wormhole, meanwhile the Doctor and Christina have a look around, they see a crashed Tritovore spaceship, with two survivors. They decide to work together, but some of the swarm are on their ship, and both Tritivore soon become a meal for the swarm.
They leave the ship with some anti-gravity equipment, fasten it to the bus and sail for the wormhole, pursued by the swarm. They arrive back on Earth, Doctor Malcolm closes the wormhole, but three swarm get through and are killed by UNIT troops.
The Doctor is told by a passenger your song will end soon and He will knock 4 times, Christina escapes the law on the flying bus, the doctor enters his TARDIS!