The murder of a police informant prompts 15 Division to investigate the notorious East Jameson gang, while Gail protects the only witness to the crime.
The TARDIS lands on the English coast in 1066, but the time travellers are puzzled to find modern day artefacts there. A monk observes the arrival of the TARDIS, but seems unsurprised.
The Doctor is held as a prisoner by the Monk in the monastery, whilst Vicki and Steven are searching 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 certain plans for the Viking invaders. Returning to the Monastery, Steven and Vicki make an amazing discovery.
Discovering the Monk's plan, the Doctor is determined to stop him from meddling with history. As the Viking fleet begins their approach, time is running out for both the Monk and the Doctor.