Vicki and The Doctor are discussing Ian and Barbara’s departure, and he admits he will miss them. They hear a noise in the TARDIS and discover the astronaut Steven Taylor (Peter Purves) whom they had thought killed on the planet Mechanus. The TARDIS lands on the Northumbrian coast. The locals think the TARDIS is a shipping crate washed up on the shore, but they are also being studied by a man in monk’s robes (Peter Butterworth). The time-travellers find a Viking helmet and the Doctor decides it is the Eleventh Century, though Steven considers this crazy, especially after he discovers a modern wristwatch. But the Doctor, talking to villagers, realizes it is 1066 and the Norman Invasion is about to take place. Suspicious of the chanting from the local monastery, however, he investigates and discovers that the plainsong he hears is a gramophone recording. He is captured by the monk, put in a cell, and the monk makes him breakfast in the morning with an electric toaster.
Vicki and Steven are captured by villagers, who accuse them of being Viking invaders, who have already come ashore, but when they discover their mistake, they tell them The Doctor has gone to the monastery. Vicky and Steven investigate as well and realize that the monk is holding The Doctor prisoner. The village is attacked by Vikings and suffers serious losses. Vicky and Steven break into the monastery and find the recording machine, but The Doctor is gone.
The monk finds The Doctor gone too. He is back in the village, in fact, hears about the Vikings’ arrival, and realizes the monk is making plans to interfere in the Battle of Hastings. The Doctor returns and captures the monk. Vicky and Steven find that the TARDIS is gone and there are gun emplacements aimed out to sea. Two Vikings seize control of the monastery but are knocked out by the monk and The Doctor. Vicky and Steven discover the monk’s TARDIS, disguised as an altar. He has artefacts looted from every era, including modern weapons. The Time Meddling monk wants to change the result of the Battle of Hastings, so he can become the power behind King Harold.
The time travellers are trussed up in the monk’s TARDIS and he sets out with his Viking cronies. They are attacked by the Saxon villagers, who free the time travellers. The Doctor rigs the monk’s TARDIS, stealing a vital circuit. His own TARDIS reappears undamaged after the tide goes out. The monk returns to his TARDIS to find it miniaturized and useless.
The only surviving print of Episode Four was recovered from an overseas broadcaster. Twelve seconds of footage depicting the brutal slaughter of the hapless Vikings had been excised. Several moments were restored later by CGI. Clearly, the monk is another renegade Time Lord from Gallifrey; the console in his Tardis is identical to the one we know. One wonders why The Doctor would not steal a chameleon circuit while he was at it, but of course the Blue Box was already a beloved character in the series. And one can imagine what the bean-counters at the BBC would have thought of building a new TARDIS for every story.