The First Doctor and Vicki are surprised to find Steven Taylor aboard the TARDIS. In a disorientated state on Mechanus, he stumbled aboard the ship and has stowed away. They are grateful he survived the collapse of the Mechanoid city and help nurse him back to health, but when the TARDIS lands on a rocky beach and they all step outside Steven takes some convincing that the TARDIS has really been able to travel in space and time.
They have in fact arrived in 1066 on the coast of Northumbria, and their arrival has been witnessed by a Monk who does not seem fazed by the materialisation. The TARDIS is soon after spotted by a Saxon villager called Eldred who runs to tell the headman of his village, Wulnoth, about it. The Doctor establishes the century from a discarded Viking helmet and heads off to the village while Steven and Vicki explore the cliffs above. The Doctor encounters Edith, Wulnoth’s wife, and convinces her he is a harmless traveller while probing her for more information. He soon finds out it is 1066, since Harold Godwinson is on the throne and has not yet faced Harold Hardrada at Stamford Bridge let alone William the Conqueror in the Battle of Hastings. He then turns his attention to the nearby monastery, at which monks are chanting despite only one of them ever being seen, especially after the chanting seems to slow down as if played back from a recording at the wrong speed. He determines to visit the building. When he gets there the Monk lets him in without revealing himself and then allows the Doctor to prowl around. He finds a gramophone playing the monastic chanting, and the Monk also has modern conveniences such as a toaster and a manufactured teapot. The Monk soon has the upper hand and manages to trap the Doctor in a makeshift cell.
Steven and Vicki have meanwhile encountered Eldred and noticed his possession of a wristwatch that the Monk dropped earlier. They spend the night in a clearing and the next morning head off back to the TARDIS, little realising Wulnoth has overheard them. Within minutes they are ambushed by the Saxons and taken to the village council. After a heated discussion they convince Wulnoth they are but travellers and are given some provisions to travel on, though Vicki is equally heartened to hear from Edith that the Doctor passed by her hut on his way to the monastery. Steven and Vicki decide to visit the monastery next to try and find their missing friend. The Monk tries to dissuade them from entering but gives himself away deliberately by describing the Doctor too accurately, and so Steven and Vicki decide he must be a prisoner inside the monastery. They decide to break in after dark, which delights the Monk as he prepares the same trap for them that caught the Doctor.
The Monk has meanwhile been surveying the seas with binoculars and is pleased to finally sight a Viking ship on the horizon. Soon the Vikings land and two small groups are sent to search the area, with one group of three heading toward the Saxon village. One of the Vikings finds and attacks Edith, leaving her traumatised, and in response some of the Saxons go hunting for the invaders. The three Vikings are drunk when they are found and the giant that attacked Edith is cut down, though his companions Sven and Ulf manage to flee. Eldred too has been badly wounded and Wulnoth takes him to the monastery for help.
At the Monk’s lair Steven and Vicki have stolen in under cover of darkness. They too find the gramophone and are stunned. The Monk has his trap prepared but cannot spring it due to the arrival at the door of Wulnoth and the injured Eldred, whom Wulnoth insists be taken into the Monk’s care. Steven and Vicki have meanwhile found the cell empty bar the Doctor’s cloak and they then manage to leave the monastery via a secret passage.
The Doctor has actually taken the same passage himself and returns to Edith in the Saxon village. He soon hears of the Viking invasion scouting party and, upon leaving Edith’s house, decides to head back to the monastery to track down Steven and Vicki, having learned they have gone there. Steven and Vicki have meanwhile found to their dismay that the TARDIS has been submerged beneath the incoming tide. Afraid that the Doctor may have had to leave in it, they resolve to check for him at the monastery anyway, especially after they discover an atomic bazooka trained out to sea from the clifftop near where the TARDIS was.
The Monk is intent on using the Vikings for his own ends and, once Wulnoth has departed his monastery, produces an elaborate checklist that builds to a meeting with King Harold himself. There is another knock at the monastery door and this time it is the Doctor who has the upper hand when the door is answered. Fooled into thinking he is being held at gunpoint, the Monk is marched back inside and is about to answer a few questions when there is yet another knock at the door. When the Doctor and Monk answer, they are overpowered by the two Vikings, Sven and Ulf. In the ensuing confrontation the Monk is able to slip away, leaving the Doctor as the Viking prisoner. It is a state of play that does not last long. The Doctor knocks out Sven and elsewhere the Monk does the same to Ulf and securely ties him up.
The Monk uses his freedom to persuade the villagers to light beacon fires on the cliff tops, lying that he is expecting materials by sea to enhance the monastery, when in fact he wishes to lure the Viking fleet to land nearby. Wulnoth says he will light the fires, but does not do so as he realises the danger.
Steven and Vicki return to the monastery via the secret passage and investigate the crypt, where a heavy power cable emanates from a sarcophagus. When they look inside, they discover that it is a TARDIS of the Monk's very own – he must come from the same place as the Doctor (though the term Time Lord is not used). The Monk has meanwhile returned to the monastery and is once more under the Doctor’s control. He reveals his plan is not to help the Vikings but to lure them to the coast where he hoped to destroy the invasion fleet with atomic bazookas. This would prevent the Viking invasion and thereby shore up King Harold to such an extent he would not then lose the Battle of Hastings. In short, the Monk is a Time Meddler who left his and the Doctor’s own planet some fifty years after the Doctor himself. Steven and Vicki have found further evidence of his meddling in his TARDIS: a journal recording his meeting with Leonardo da Vinci to discuss powered flight, providing anti-gravitational discs to help the ancient Celts build Stonehenge, and using time travel to collect a fortune in compound interest from a bank. The Doctor denounces the Monk for seeking to alter history and forces him to reveal his TARDIS, where they find Steven and Vicki. Together the time travellers piece together the Monk’s immoral plot, which the Monk insists is intended to stabilise England and benefit Western civilisation.
The Vikings have meanwhile freed themselves from their bonds and decide to avenge themselves on the monks who have imprisoned them. Eldred spots them and, despite his injuries, flees to the village where he raises Wulnoth and a squad of Saxons to deal with the marauders.
At the monastery the tables have turned. Ulf and Sven have formed a contrived alliance with the Monk and have tied up the Doctor’s party while the three of them take the bazooka shells down to the cannon on the beach. The scheme is foiled however when the Saxons arrive and engage the fleeing Vikings in a nearby clearing, killing Sven and Ulf in battle.
The Monk hides while this fighting rages, little knowing that the Doctor and his friends have been freed and are tampering with his TARDIS. With his scheme in ruins, the Monk decides to leave and returns his TARDIS, though the Doctor has gone and left a note assuring the Monk his meddling days are ended. When the Monk looks inside his TARDIS he realises the Doctor has taken the dimensional control and the interior of his ship has shrunk beyond use, leaving him stranded in 1066 with an angry band of Saxons nearby. The tide having gone out, the Doctor and his friends are free to leave this primitive time in their TARDIS, and journey onward to the stars.