The Prehistoric Monster

The Doctor, Ian and Barbara exit the TARDIS with the Doctor telling them that they're in the prehistoric times. Later on they realise that a creature is entering the TARDIS. Realising the chaos it could cause, they rapidly make their way back to the ship, however just ouside the doors they realise that they need to enter with the creature if they are to ever have a chance at leaving. The Doctor moves the TARDIS to another time and the creature begins shrinking until it becomes nothing. The Doctor reveals that the creature became extinct as they moved into the future.