The story opens in Stockholm in July 1917, a city teeming with spies, refugees, and profiteers. Baron Fayerling and his associates are working for the mysterious Gabriel Ortiz, an adventurer orchestrating a vast and sinister plot. Simultaneously, Maurice Wallion, a perceptive journalist known as the problem-hunter, begins tracking down the dark forces gathering in the capital. Leonard Grath, a young and carefree artist who has just returned from California, discovers that his ancestral home, the Copper House, has been leased and nearly bought by mysterious foreigners.
Ignoring Wallion's warnings, Grath travels out to the estate, only to find himself treated as an unwelcome intruder in his own house. He encounters the enigmatic Bernin family, consisting of Lona Ivanovna, her niece Sonia, and an invalid relative. Behind the closed doors of the Copper House, a high-stakes struggle is taking place.
Bernard Jenin, actually Sergius Tassler, has fled from Russia with Tarraschin's memorandum—a dangerous document containing plans for a monarchist counter-revolution that could compromise major European statesmen. Ortiz and his agents are desperate to recover the paper to cement their control over the impending chaos. As the mansion is placed under armed guard by Rastakov and the baron's henchmen, Wallion also infiltrates the estate with the help of an Austrian detective disguised as a gardener. The tension escalates as the characters navigate layers of deception, disguises, and shifting loyalties.
The climax arrives with the dramatic arrival of Gabriel Ortiz via a stolen submarine during a fierce summer storm. The mansion is entirely isolated, and the conspirators corner the inhabitants, demanding the surrender of the memorandum. Despite their desperate attempts to escape and summon help, the protagonists are captured. The resolution hinges on a tense confrontation in the dining room, where Ortiz unmasks Sergius and closes in on the missing document, bringing the global conspiracy to a head within the walls of the isolated estate.
