Somebody's interfering with time. The Doctor, Rose and Captain Jack arrive on modern-day Earth to find the culprit -- and discover a Neanderthal Man, twenty-eight thousand years after his race became extinct. Only a trip back to the primeval dawn of humanity can solve the mystery. Who are the mysterious humans from the distant future now living in that distant past? What hideous monsters are trying to escape from behind the Grey Door? Is Rose going to end up married to a caveman?