The Doctor steps aboard a double decker bus as it does its normal journey. On board is Lady Cristina de Souza a big time jewel thief amongst others. As they go through a tunnel, it enters a wormhole and takes them to another world, San Helios, a desert planet. The top of the double decker is mangled caused by the pressures of the wormhole. (In truth, the double decker was damaged en route to where they were filiming the scene.
On Earth, U.N.I.T. led by Capt. Magambo is called into help with the aid of Professor Malcolm Taylor (played by British comedian, Lee Evans) who have cornered off the area to prevent anyone else having the same fate. Taylor sets to work out the solution to the problem. He is assisted in calculations by a deep space link with The Doctor.
On the planet, Cristina decides to take charge before The Doctor can suggest anything. However, the Doctor does recommend actions which Cristina wholeheartedly agrees with. A number of the passengers start digging out the wheels whilst he goes off to investigate. The Doctor meets the insectoid Tritovore who explains everything to him. The inhabitents of the planet and Tritovore were trading partners until the stingray swarm came and attacked the planet. San Helios was an advanced planet of some billions of people. However the swarm attacked and turned the whole planet to a wasteland. The stingrays open portals to other planets and once they finished with devouring one planet, they move onto the next. The next planet the Swarm was to attack was Earth unless the portal can be closed.
When the Doctor returns to the bus, the wheels are freed and they are able to take off and return to Earth. Once through the other side, Malcolm is able to close the portal. de Souza is arrested but the Doctor against all that he stands for, frees her from her cuffs and lets her escape.
It was the first of a few special edition episodes as David Tennant, the Doctor wasn't able to commit to a new full series that year.