In this special, Rose (Billie Piper) drops in on her mother and boyfriend in the TARDIS - accompanied by a complete stranger, the new Doctor Who (David Tennant). Disoriented and weak from the regeneration process, the new Doctor slips in and out of consciousness as Rose desperately tries to revive him. Meanwhile, Prime Minister Harriet Jones has just been informed that a British space probe, on its way to Mars for a Christmas Day landing, has gone missing. It has been kidnapped by a monstrous race known as the Sycorax, who are set on taking over the world. Only the Doctor can stop their terrifying advance.