Following a dramatic rock fall at a South African gold mine, the rescue party, headed by Rod Slater, (Roger Moore) are puzzled to discover the body of General manager Frank Lemmer - a veteran miner - in the prohibited Big Dyke area of the mine. Meanwhile, in London, the mine's managing director, Manfred Steyner, is hatching a plot against its owner Harry Hirschfeld (Ray Milland) with English financier Farrell (John Gielgud). There is a vast underground lake beyond the Big Dyke, which if released would flood and destroy the gold field, raising the world price of gold. This is the project Lemmer was involved in when he met his death. Slater must now be persuaded - innocently - to take his place and is shown a doctored copy of a geologist's survey showing a rich seam of gold beyond the Big Dyke, which Steyner encourages him to excavate. Slater encounters a plot of deception and intrigue, as well as finding romance in the form of Steyner's beautiful wife Terry (Susannah York). Slater is called on to save the mine and its workers when the conspirators try to blow it up.