A burglar named Lady Christina de Souza (Michelle Ryan) steals a gold chalice once owned by King Aedelstan from a London museum and tries to escape on a double-decker bus. The Doctor comes on board, sight-seeing. The bus passes through a wormhole and ends up on the sands of the planet San Helios. Of course, it does. The Doctor studies the wormhole and realizes they should all be dead, but the bus acted like a Faraday Cage and protected them. Unfortunately, the driver (Keith Parry) steps through the wormhole and his skeleton appears in London. UNIT is called. Captain Erisa Magambo (Norma Dumezweni) and scientific advisor Malcolm Taylor (Lee Evans) investigate.

The Doctor is able to contact UNIT to co-ordinate returning the passengers to Earth. The Doctor and Christina scout ahead while the others try to repair the bus. They spot a sandstorm coming and are met by two Tritovores—giant flies wearing coveralls—who take them to their wrecked spaceship. They explain that San Helios used to be inhabited by billions but has become desert. They scan the approaching storm and find it is a swarm of stingray-like aliens that are devouring the ecosystem. The Doctor thinks they are generating the wormhole to travel to their next planet to feed upon. Christina uses her burglary skills to get her hands on a power crystal from the Tritovore ship, but accidentally wakes a stingray, which consumes the Tritivores.

The swarm almost upon them, the Doctor uses the crystal’s mount, which is an anti-gravity device, to enable the bus to fly. He has to take the chalice that Christina stole and hammer it down to interface with the bus’s controls. They fly back through the wormhole just before Malcolm and Magambo shut it down, but three stingrays sneak through, which UNIT manages to destroy. Christina is arrested. She asks the Doctor to take her with him, but he refuses. However, he arranges for her to escape and fly off in the bus. A passenger named Carmen (Ellen Thomas), who is psychic, tells him fate will knock four times before taking him.

The episode had a joyous feeling, instead of the bittersweet feel of the last several episodes. Michelle Ryan was rumoured to be in line to play a long-term companion for Matt Smith, the 11th Doctor. The character she plays is upper class, but an adventurer. Her verbal sparring with the Doctor is a hoot. Professor Taylor, the UNIT scientist, becomes a kind of loving caricature of Doctor Who fans, his humour contrasting with the UNIT Captain’s no-nonsense seriousness. The story was influenced by Indiana Jones, Flight of the Phoenix, and Pitch Black, mixed up with Charade and Topkapi. It was filmed in Dubai and Cardiff. The first high-definition Doctor Who story, it got mixed reviews.

The bus was damaged at the Port of Dubai and they wrote the damage into the plot and smashed the bus in Cardiff the same way. Of course, there was a sandstorm, as George Lucas could have predicted. After all this, many people thought the desert was fake. It was the 200th Doctor Who story, and the number of the bus was 200. Adam James, who played a frustrated detective, D.I. MacMillan, is the godson of Jon Pertwee and an old friend of David Tennant. There had been plans for a Doctor Who/Star Trek Enterprise crossover (really), but the latter series was cancelled. Having spent the previous six months playing Hamlet in Stratford-upon-Avon, David Tennant had to study his own Doctor portrayals to get the proper accent back. This romp is the 10th Doctor’s last happy ending.