UNIT troops transporting a missile encounter difficult and freak conditions, like some kind of interference. Halting at an archaeological site they become caught up in whatever forces are gathering. Explosive like sounds hearld the arrival of what appear to be knights in armour. However, they are carrying weapons from a different time.

A strange signal brings the TARDIS to same area and time: and an encounter with one of the Knights suggests that the Doctor may once have been or will be Merlin. the nearby dig site is believed to be in the area where legend has it King Arthur was buried. As knights from another dimension begin their battle, UNIT calls in the assistance of an old solder - Brigadier Lethbridge-Stewart.

Towards the end of the 19th century, at the house of Gabriel Chase in the suburb of Perivale, Reverend Ernest Matthews has come to see Josiah Smith. Smith espouses Darwin's theory of evolution, and Matthews has come to challenge his Godless views. The staff are surprised that he has arrived at this time, no-one local stays at the house when night falls. As six in the evening draws nearer, the staff make a hasty exit. As darkness falls though, the house appears to come alive. Maids emerge from secret panels and Josiah bids his ward, Gwendoline greet their guests.

Ace emerges from the TARDIS and tries to determine from the objects around her where and when they have arrived. Victorian Earth it appears, the Doctor informs her that it is a surprise. As they wander through the house they encounter a man wandering around: he is searching for Redvers Fenn-Cooper. As the search contiues, the man stares at his reflection "ah, there you are, old chap. Redvers, I found you." What could have affected his mind? And what is locked away in the spaceship in the cellar? And has the Doctor brought Ace to the house from her childhood? One of the few places that scared her because she sensed the evil permeating its ruins.

Off the coast of Britain during World war 2, two dinghies of Russian soldiers are rowing thrugh the fog. Only one dinghy makes it, the other presumably claimed by the undercurrents. Despite their stealth, the Russians are expected: Commander Millington at the nearby base is leading a scheme that will involve allowing the Russians to capture the Ultima machine, a code-cracking device being operated by Doctor Judson. Although the Russians are currently allies of Britain, it is suspected that this alliance will only last for the duration of the war. The moment the Nazi war machine is beaten or turns its attention away from Russia...best to be prepared for that eventuality.

The Doctor and Ace arrive at the base: initially they go unnoticed until the observers conclude that these two people are not the house guests they were ordered to expect. As the soldiers close in on them, the Doctor types up a letter which appears to accord him special access. This seems to satisfy Millington and so he shows the Doctor and Ace around the base, giving them a brief glimpse of his plans. It appears that both Millington and Doctor Judson have a common outside interest involving local legends of a Viking curse and Viking runes. Could there be some truth to the legend that something lurks in the seas nearby?

What starts off as just another day in Perivale soon changes when something appears out of nowhere and pursues a man out washing his car. Man and pursuer vanish. There have been other disapperances as Ace and the Doctor learn: including some of Ace's old friends. Yet Perivale still seems a quiet and dull suburb. While the Doctor becomes intrigued about the local cats, Ace wanders off to see some of the places she used to hang around. At the local youth centre, ex-army sergeant Paterson (the sarge) is teaching some of the local lads survival skills. His philosophy is perhaps a bit more than some are prepared to accept - nature does not make allowances for codes of conduct: kill or be killed. When Ace disappears, the Doctor sets out to find her. The sarge has been alerted by other members of the neighbourhood watch about a suspicious man lurking around - and finding the Doctor stalking a cat he tries to apprehend the Doctor. Seconds later, both the Doctor and the sarge begin to learn what happened to all the vanished people.