Doc Savage is aboard ship on a trans-Atlantic cruise to England to fill a short lecture engagement before the Fellowship Of Scientists. Instead of waiting, Johnny decides to look into the strange matter alone and is not heard from after exploring the marsh.

A few days later, Doc arrives in Southampton with Monk and Ham. An unscrupulous detective named W.P. Wall-Samuels, says he was hired by Johnny. He informs Doc that Johnny is trailing some unknown person who boarded a steamer for South America.  Wall-Samuels is really working for a criminal mastermind who's goal is to send Doc and his men on a wild-goose-chase. The plan backfires and Doc discovers their true motive. The mystery leads to  tiny Magna Island in international waters off the English coast and a bizarre scheme to extract gold - from seawater!