Hiller, a computer expert, was bribed by group of bank robbers to obtain details of the security system at a newly-built bank. Having obtained the information, he thought he'd seen the last of the robbers. But now they've traced him and his son to London. They hold the son hostage and force Hiller to decode the information about the alarm and then to take part in the robbery.

A chance discovery leads American mining engineer Ben Harris and acquaintance Harold to discover a lost city under the sea while searching for their kidnapped friend Jill. Held captive in the underwater city by the tyrannical Captain (Vincent Price), and his crew of former smugglers, the three plot to escape...