The Waters of Mars

The Doctor arrives on Mars in the year 2059 and joins the first human Earth colony led by Adelaide Brooke. The day he arrives is also the day history records the entire settlement destroyed in an unknown cataclysm with all members of the settlement being killed. Everyone soon realizes that colonists are being infected by some unknown agent and are being transformed into zombie-like creatures. For reasons that are eventually explained, The Doctor is not prepared to save them and tells Brooke that she has to die. He quite torn by this decision and eventually chooses another path, one that will have other repercussions for him in the future.