A group of Russian soldiers paddles dinghies toward the Northumbrian coast. Something stirs beneath the water and one of the Russian boats disappears in the mist.
The Doctor and Ace land nearby on the grounds of a secret naval base. They are eventually surrounded by troops, but the Doctor bluffs his way through to Dr. Judson's office.
Captain Sorin, the leader of the Russians, commands his soldiers to speak English. They find one of the missing soldiers on the beach, pale and incoherent.
The Doctor bursts into the office of Judson, a wheelchair-bound cryptographer. The Doctor quickly forges a letter of authority for himself and Ace.
Sorin waits for dark, leaving one man to guard the beach. The guard finds a packet of orders than includes information about Judson. He is then attacked by something.
The next morning, Reverend Wainwright concludes his sunday service. The Doctor arrives, looking for Judson. Ace meets two young women, Jean and Phyllis, who have been evacuated to the country and billetted with the severe Miss Hardaker. The girls agree to meet later at Maiden's Point.
As Wainwright guides the Doctor into the crypts, he tells Ace that the church is supposedly cursed, being built on Viking graves. They find Judson trying to translate 9th century Viking runes. Ace can hear the sound of machinery nearby but the Doctor dismisses it as the church organ. As they leave, the Doctor finds subsidence near some of the ancient graves and decides to accompany Ace to Maiden's Point.
Mrs. Hardaker is furious when Jean and Phyllis tell her their plans, Maiden's Point being a place of ill repute.
Ace finds the Russian orders on the beach. She suggests warning the base, but the Doctor thinks they already know. He heads back to the church, warning Ace not to go into the water.
Judson works to crack German ciphers. Millington, the base commander, assures him they only be cracked by thinking like the Germans.
The Doctor finds Wainwright perusing an old tome that has translations of the inscriptions made by Wainwright's grandfather.
Phyllis and Jean arrive at Maiden's point and take a swim. Ace refuses to join them in the water.
The runic translation tells of a group of vikings returning with eastern treasures. But a curse followed them. Many warriors were killed by the curse and they took refuge in Maiden's Bay. He finds an eerie similarity to the order given to the Russians.
As Phyllis and Jean relax on the beach, they find crushed bits of metal. They just avoid running into the Russians.
The Doctor brings the translation to Judson. Judson reads of a menace under the sea. As he does, a Russian soldier flings a piece of metal into Maiden's Bay. A repitilian claw catches it. Within a submerged viking ship are the corpses of the missing Russian soldiers.
The Doctor and Ace find a transcription room filled with secretaries. One, Kathleen Dudman, has a baby. Ace is fascinated until she learns it has the same name as her mother. Millington comes in and demands that Dudman remove the child from the base.
The Doctor finds Millington's office. It is a near-perfect replica of the German cypher room in Berline. Ace finds a chess set carved in Viking images.
Judson shows the translation to Milllington, who reads of the world ending and the Wolves of Fenric returning to do battle with the gods.
The Doctor and Ace return to the bay and find the dead Russian soldier. But they are quickly surrounded by Sorin's troops, who level their rifles at them…