Mason Valentin was a 15-year-old human. One day, he felt lovesick toward his fellow student, Ana Comparetto and couldn't bear to be in school that day. Mason discovered a fleet of vans deliver a small gift to everyone in his area. He disobeyed the order not to open until Christmas, and found a shrunken, amnesiac Tenth Doctor inside the box.
Noticing Ana, who lived across the way, had brought her gift indoors, the Doctor and Mason went up to her house to investigate the gifts. The Doctor found an Ulian robot inside and prematurely activated it. The robot attacked, mistakenly bringing Ana on board with its transmat gun. The Doctor quickly deactivated the robot with his sonic screwdriver and the Doctor and Mason used the robot's "magic wand" to fire a transmat beam to board the ship.
On board the ship, the Doctor released Ana from her cell. The Doctor had remembered why he was in the box: he had been searching for the missing Princess Cilia and upon discovering her ship, the robots attacked, unwittingly shrinking him and placing him inside one of the packages, which the robots had been using to find Princess Cilia, whom they only knew was somewhere near a Christmas tree. The Doctor told the robots that Ana wasn't the princess, and though their methods of tracking the princess were scrambled, he could find her easily. The Doctor was brought back to full size and returned Mason and Ana to Earth in the TARDIS. The Doctor discovered Cilia in one house disguised as a fairy on a Christmas tree. Ana told Cilia the war was over, and the Doctor told her to order the robots to send her back to Ulian Alpha.
Asking what he'd do now, the Doctor mentioned he was avoiding something, and had the realisation: "I'm the Snowman and...time's up." Mason watched as the the Doctor departed in the TARDIS. (PROSE: The Doctor on My Shoulder)