The Doctor and Ace are on the run from Kryian rocket pods. They make it to the TARDIS, but with its doors open, some of the energy from the pods makes its way into the TARDIS, causing damage.
The TARDIS dematerialises, and Ace steps out to find herself in Victorian London on Christmas Eve, in 1889. She wants to get a Christmas dinner, but she can't get the Doctor to agree that tomorrow is Christmas. When they dematerialise again, they are in Los Angeles in 2023 — again on Christmas Eve. The Doctor admits that the damage to the TARDIS allows them to move through years but not days; in other words, anywhere they go it will be Christmas Eve. Ace feels like she has won her point.
Virginia puts on her environment suit and leaves the dome to do her chores. She sees two odd shapes, and when she runs away she falls, tearing her suit. The Doctor and Ace, wearing personal force fields, take her back to the dome. The people in the dome are surprised to find that the Doctor and Ace aren't telepathically connected.
While the Doctor speaks to the village's elders and obtains the part he needs for the TARDIS, Ace learns from Virginia's mother, Belen, that the Earth was invaded by the Huyfbfgriniotites eighty-nine years ago. The Huyfbfgriniotites enslaved the people and stripped the planet of natural resources, then left. As far as Belen knows, they are the only survivors. Ace is horrified to learn there are only fifty-three of them.
Ace tells Virginia about Christmas, and through their telepathic link the other survivors have learned about Christmas too. They think this will be their salvation — they need food and medicine. They perform the Christmas rituals, such as decorating a "tree" and changing the pattern of the searchlight to look like a star.
Ace is appalled that the survivors believed her story, and she wants the Doctor to do something about it. When he refuses, she grabs the TARDIS part from him in frustration and receives an electrical shock that knocks her out.
When Ace comes to, she expects the survivors to be angry. However, they are happy — Santa Claus came. The children have toys, and the adults have food and medicine. The Doctor and Ace head for the TARDIS. On their way, they encounter several other humans — they've been alerted to the presence of survivors here because they saw the searchlight shaped like a star. It turns out that the Doctor knew all along that there were other survivors on Earth.
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