The episode begins with a group of armed guards surrounding a gold chalice once belonging to King Athelstan; Christina then descends on a winch, avoiding the blue laser beams protecting the cup. She then evades the police by riding on the same London bus as the Doctor, shortly before the bus, now pursued by the police, suddenly passes through a wormhole and arrives on the desert planet of San Helios. The Doctor and the other passengers find that the wormhole is still present, but deduce the bus had protected them like a Faraday cage after the bus driver is killed trying to cross back. The driver's skeleton coming out of the portal causes the police to call in UNIT, commanded by Captain Erisa Magambo (Noma Dumezweni) and aided by scientific advisor Malcolm Taylor (Lee Evans), to close the wormhole. Trapped on a heavily damaged bus, the other passengers introduce themselves: Angela (Victoria Alcock) is a middle-aged mother travelling home; Lou (Reginald Tsiboe) and Carmen (Ellen Thomas) are an elderly couple who win £10 each time they play the National Lottery due to Carmen's low-level psychic abilities; Barclay (Daniel Kaluuya) was travelling to a friend's house to ask her on a date; and Nathan (David Ames) was travelling home to watch television. The Doctor and Christina decide to scout the planet and an approaching sandstorm while Nathan and Barclay try to fix the bus.
The Doctor and Christina encounter the Tritovore, an anthropomorphic fly species, who take them to their wrecked spaceship. The Tritovore explain that they were making a routine goods collection from the planet but crashed in an unfamiliar environment; a year previously, the planet housed a hundred billion inhabitants and a thriving ecosystem. The Tritovore send out a probe to investigate the cause, and discover a large swarm of metallic stingray-like aliens, who routinely create wormholes and destroy ecospheres as their biological imperatives. To rescue the Tritovore and the bus passengers, Christina uses her burglary skills to retrieve a crystal which powers the spaceship (together with the pedestal it is located on), unintentionally awakening a stingray that kills the Tritovores.
The Doctor attaches parts of the pedestal to the bus and uses the chalice of Athelstan as an interface to the technology. The technology allows the bus to fly through the wormhole, with the stingrays in hot pursuit. As soon as the bus—and three stingrays—travels through the wormhole, Malcolm closes the wormhole. After UNIT dispatch the stingrays, Christina asks the Doctor to travel with him; he rejects her because he does not want to lose another companion.
At the end of the episode, the characters part ways. The Doctor recommends that UNIT hire Barclay and Nathan, Christina is arrested by the police for the theft and Carmen has a premonition that visibly shocks the Doctor:
You be careful, because your song is ending, sir. It is returning, it is returning through the dark. And then... oh, but then... he will knock four times.
As a final act of kindness, the Doctor uses his sonic screwdriver to release Christina from her handcuffs. She flies away in the bus as he enters his TARDIS and dematerialises.
Doctor David Tennant (Tenth Doctor)
Companion Michelle Ryan (Lady Christina de Souza)
Lee Evans – Malcolm Taylor
Noma Dumezweni – Capt. Erisa Magambo
Adam James – D.I. McMillan
Glenn Doherty – Sgt. Dennison
Victoria Alcock – Angela Whittaker
David Ames – Nathan
Ellen Thomas – Carmen
Reginald Tsiboe – Lou
Daniel Kaluuya – Barclay
Keith Parry – Bus driver
James Layton – Sgt. Ian Jenner
Paul Kasey – Sorvin
Ruari Mears – Praygat
Writer Russell T Davies and Gareth Roberts
Director James Strong
Script editor Lindsey Alford
Producer Tracie Simpson
Executive producer(s) Russell T Davies
Production code 4.15
Series 2009 Easter special
Length 60 minutes
Originally broadcast 11 April 2009