Doctor: David Tennant (Tenth Doctor)
Companion: Lindsay Duncan (Adelaide Brooke)
Peter O'Brien – Ed Gold
Aleksander Mikic – Yuri Kerenski
Gemma Chan – Mia Bennett
Sharon Duncan Brewster – Maggie Cain
Chook Sibtain – Tarak Ital
Alan Ruscoe – Andy Stone
Cosima Shaw – Steffi Ehrlich
Michael Goldsmith – Roman Groom
Lily Bevan – Emily
Max Bollinger – Mikhail
Charlie De'Ath – Adelaide's father
Rachel Fewell – Young Adelaide
Anouska Strahnz – Ulrika Ehrlich
Zofia Strahnz – Lisette Ehrlich
Paul Kasey – Ood Sigma
Directed by Graeme Harper
Written by Russell T Davies and Phil Ford
Script editor Gary Russell
Produced by Nikki Wilson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.16
Series 2008–2010 specials
Length 60 minutes
Originally broadcast 15 November 2009
The Doctor arrives on Mars via his TARDIS in 2059, near humanity's first Martian colony, "Bowie Base One". He dons a spacesuit to walk to the base, where inside he is detained by its crew led by Captain Adelaide Brooke. As they interrogate him, he discovers that today is a "fixed point" in time; the base will explode, which will spur humanity to explore the stars. He tries to stay uninvolved, but Adelaide forces him to assist her responding to an alert from the remote biodome.
Two crewmen appear to be in a zombie-like state, generating copious amounts of water. One of Adelaide's team gets infected, and they quarantine her while sealing off the biodome. From observation, they discover that the infection is by the intelligent virus, the Flood, and now seeks to get to Earth. The Doctor suspects the Flood were sealed in the glacier the base was using for water by the Ice Warriors aeons ago, and a bad filter allowed the virus to infect the crew. Adelaide realises the remaining crew have not yet touched that source of water and are uninfected, and orders the crew to evacuate to their rocket back to Earth while setting the base to self-destruct. Before departing, the Doctor is forced to explain to Adelaide what he knows happens why he cannot get involved.
The Doctor starts walking back when he hears cries of help: the infected crew have broken the biodome seal and are now attacking the central hub with massive amounts of water, infecting several others. The rocket's pilot is infected, but he causes the rocket to self-destruct to strand the Flood. The Doctor races back and rescues Adelaide and the last surviving crew, Yuri and Mia, via the TARDIS moments before the base destructs. He returns them to Earth, arriving outside Adelaide's home. Yuri takes a shocked Mia away, while Adelaide demands to know why the Doctor saved them and risked changing history. The Doctor insists that he can break the laws of time to save the "little people", calling himself "Time Lord Victorious". Adelaine becomes indignant and warns him, "I don't care who you are - the Time Lord Victorious is 'wrong'", before stepping into her home and killing herself.
The Doctor is shocked; he realises that history has not changed save for knowledge of the base's fate from Yuri and Mia. Ood Sigma appears in the street. Visibly shaken, the Doctor sees this as a message and says, "I've gone too far", and asks if it is time for him to die. Unresponsive, Sigma vanishes, and the Doctor staggers back into the TARDIS to the ringing of the cloister bell. With a defiant "No!", he begins to operate the controls.