The TARDIS materialises at a secret naval base off the coast of Northumberland toward the end of the Second World War. Dr Judson, a scientist there, has created the Ultima Machine, an early computer designed to break German codes. The base's Commander Millington plans to let a Russian commando unit led by Captain Sorin steal the Machine's core, which he has booby-trapped with deadly toxin.

Judson uses the Machine to translate some ancient runes from the crypt of the nearby St Judes church and this leads to the release of Fenric, an evil entity from the dawn of time whom the Doctor trapped seventeen centuries earlier in a Chinese flask by defeating it at chess. The flask was later stolen and buried at the church by Vikings.

The base and church are attacked by Haemovores. These are humans who have been transformed into hideous vampiric creatures by the Ancient Haemovore - the last survivor of a pollution-ravaged future Earth, who has been brought back in time by Fenric. Fenric takes over Judson's body to challenge the Doctor to a rematch at chess, and Ace unwittingly helps it to win.

Fenric, now in Sorin's body, reveals that Ace, Judson, Millington, Sorin and Wainwright, the vicar of St Judes, are all 'Wolves of Fenric' - pawns in its battle against the Doctor. It now plans to release the deadly toxin but the Doctor succeeds in turning the Ancient Haemovore against it and its host body is killed by the gas.

The baby daughter of a young woman whom Ace helped to escape from the Haemovores is revealed to be her future mother.