The story opens in May nineteen fifteen with the sinking of the Lusitania, where a young American girl named Jane Finn is entrusted with a vital secret treaty meant to prevent disaster. Five years later, London is a place of desperation and high living costs, leading two demobilized youths, Tommy Beresford and Tuppence Cowley, to form the Young Adventurers, Limited, an agency willing to do anything for money. Their playful advertisement unexpectedly plunges them into a massive international conspiracy involving a shadowy mastermind known only as Mr. Brown.
By accepting bogus employment, Tuppence infiltrates a criminal syndicate headed by the menacing Mrs. Vandemeyer, while Tommy trails suspects through the foggy streets of London into a den of conspirators and revolutionaries. As the duo draws closer to the truth, Tommy is captured and imprisoned by the gang, barely escaping with his life after using quick wits and bluffing tactics.
Meanwhile, Tuppence manages to secure vital information through dangerous espionage within the enemy's inner circle. The quest leads them across England to find the real Jane Finn, who has been living under a severe case of amnesia since the shipwreck, having hidden the oilskin packet in a remote rocky crevice in Wales. Though they finally recover the hidden document, they discover that Mr. Brown has already bypassed them, leaving behind a mocking note. The psychological battle culminates in a race against time to thwart a general strike and an impending revolution, culminating in a devastating personal loss that tests Tommy's resolve to the absolute limit as the hidden identity of the chief mastermind finally comes within reach.
