Discovering the Monk's plan, the Doctor is determined to stop him meddling with history. As the Viking fleet approaches, time is running out for both the Monk and the Doctor.
The Doctor is held prisoner by the Monk in the monastery whilst Vicki and Steven search for him. Meanwhile, a Viking war party arrives ahead of the main invasion fleet.
Steven and Vicki discover an atomic bazooka. It seems that the Monk has plans for the Viking invaders. Returning to the Monastery, Steven and Vicki make an amazing discovery.