The Doctor is in a seasonal mood when he arrives in London on Christmas Eve 1851. There are snowflakes in the air, carol singers in the market square and a generally festive air all around. When a woman in distress calls for him to help her, he arrives to find someone else answering her call and calling himself the doctor. The alternate Doctor seems to know more than he should, dresses like the real Doctor and even has a Tardis - though in his version, it's actually a hot air balloon. While The Doctor sorts out just who this impostor is and why they find themselves in the same time and place, Mercy Hartigan and the Cybermen are kidnapping children to work in building their weapon that will allow them to crown their King.

The Doctor is in present-day London and hops on a number 200 bus that suddenly travels through a wormhole. He and the other passengers find themselves in the middle of the desert which he later determines to be on a planet in a galaxy clear across the universe. Aboard the bus one of the passengers is Lady Christina de Souza, a rather accomplished thief who was on the run after having successfully stolen a very valuable piece from a museum. She's quite enthusiastic and proves to be quite helpful as The Doctor tries to unravel what has happened to them and most importantly, devise a means of returning, given that the bus is firmly stuck in the sand and heavily damaged. Back in London, Captain Erisa Magambo has taken charge of the situation and faces the prospect of having to permanently close the wormhole before the travelers can return.

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.

Forewarned and with a sense of foreboding, The Doctor races to return to Earth fearing that The Master will somehow return. He has good reason to worry since The Masters minions have in fact found a way to reconstitute him. Re-united with Wilfred Mott, The Doctor desperately tries to locate The Master only to find that he is being held by Joshua and Abigail Naismith who are using alien technology to create a new future for Earth. It's too late however, for The Master has now turned everyone on Earth into his own image. Little does he realize the greater danger that awaits them all.

With almost everyone on Earth now recast in his image, The Master controls the world. He's shocked however when he realizes that one person hasn't changed. The Doctor soon realizes that the Time Lords are trying to return and re-establish Gallifrey. If they succeed, it is the end of time and the Doctor as we currently know him. Realizing that the Ood's prophecy will come true, he visits friends from the recent past.