The TARDIS has collided with a ship called the "Titanic". To investigate further, the Tenth Doctor runs the TARDIS's self-repair programme and boards the Titanic, which the Doctor discovers is a nuclear-powered interstellar cruiser from the planet Sto. Modelled after the Earth ocean liner of the same name, the ship is orbiting present-day Earth to observe "primitive cultures" – specifically, Christmas. The Doctor decides to stow away, and joins a party of aliens, including the waitress Astrid Peth, on a brief excursion to London. However, the populace of London fled in fear because of extraterrestrial attacks over the previous two Christmases.