The film begins with a tunnel collapse at the world-famous (but fictitious) Sonderditch gold mine outside Johannesburg, in a scene that establishes the courage of Slater and his chief miner, ‘Big King’, and the bond of trust between them. This is contrasted with the contempt with which some other white racist managers treat the black miners. It is soon revealed that the collapse was no accident, but part of a plan by a London-based criminal syndicate, which includes the previous general manager who is killed in the abortive first attempt and the mine-owner’s son-in-law and director manager Manfred Steyner, to destroy the mine so that the foreign syndicate members can profit from share-dealing and raising the price of gold on world markets. This will be done by drilling through a deep underground greenstone wall or ‘dyke’ which is all that prevents an adjacent reservoir of water from flooding the mine.
The mine’s general manager, an accomplice in the plot, was killed in the tunnel collapse. Steyner then interviews Slater, who at this stage is underground manager, for the now vacant post of general manager, although the mine owner/chairman of the board Hurry Hirschfeld (Ray Milland) has the next regular man in seniority in mind as another candidate. At this point during his initial interview, Slater first meets Steyner’s wife Terry at their luxurious mansion home and is attracted to her, but she does not at first return his interest. However, Steyner arranges for them to meet again, in the hope that Terry will influence her grandfather, the mine owner/board chairman, an old curmudgeon whom she lovingly calls “Poppsie”, in Slater’s favour. The plan works, with two consequences: Slater becomes general manager, and he and Terry start a love affair. Slater, unaware of the criminal plan, agrees to carry out the drilling but is cautious enough to plant a safety charge that will block the tunnel in case of a water leak. Steyner soon finds out that Slater is having an affair with his wife, but allows it to continue because it will keep Slater away from the mine, so that the safety charge can be disabled without his knowledge.
While Slater and Terry are holidaying together over a warm Christmas, the final breach is made in the underground dyke and a wall of an ocean of water roars into the interconnected mazes of tunnels and shafts and the mine begins to flood, trapping a thousand workers. Slater hears of the disaster on the radio news, and flies with Terry in her small private plane back to the mine making a hair-raising emergency landing on the mine access road. There is a tense scene in which Slater and Big King descend the mine, amidst rising flood waters, to repair and reconnect the electrical line to the explosive safety charge that will seal the ‘dyke’ hole. They succeed, but only because Big King sacrifices his own life to detonate the charge, letting Slater fall injured into a rubber boat in the flooded tunnel and escape. Meanwhile, Steyner is murdered by Marais, one of his accomplices, running him down with his Rolls-Royce limousine while they are observing the mine rescue operations from a nearby towering rubble slag debris hill after they hear on the radio station news reports of Slater’s set explosive charge sealing the dyke hole and saving the mine, that their plan has unravelled. Marais then also goes over the steep hill’s edge after hitting his boss, tumbling down, crashing as the car explodes, killing himself. This conveniently leaves Terry free to continue her relationship with Slater, as grandfather Chairman Hirschfeld tells him again as he is loaded into an ambulance – “Slater, you’re a maniac”!! with a smile of accepted satisfaction within a wreath of cigar smoke as the film ends.