A high-ranking member of the U.S. Secret Service, Algernon Quandy, lies in a coma in a Washington hospital, accused of stealing top-secret documents. His wife, Sophie, is suspected of being his accomplice and is therefore taken to the Hellgate prison (the same from the story Una tragica missione) to be psychically probed by Colin Randall, known as Skull the mutant with two brains who was captured years earlier by Zagor.
Skull finds no evidence of Sophie’s involvement in her husband’s alleged crime, so it is decided to bring him, under military escort, to Quandy’s bedside to extract information directly from the comatose man’s mind.
An interesting detail is that Zagor and Chico are still not back in Darkwood during this story. As mentioned by the sheriff, a letter had been sent for Zagor, but the messenger couldn't find him in Darkwood, confirming that the two heroes are still traveling back from their adventure in Saint Louis (see Huron!).
Captain McGregor and Professor Blaine, who had helped Zagor capture Skull, ask the Spirit with the Hatchet to join them on the mission to Washington with the military escort. The mutant appears cooperative, tired of years in captivity and lured by the benefits of serving the law. But as strange incidents begin to occur during the journey, Zagor starts to question whether Skull has truly changed, despite Professor Blaine’s trust in him.
The convoy is attacked by the Munsee tribe after a soldier, possibly hallucinating due to Skull’s powers, opens fire on them. Captured by the natives, Zagor escapes during the night to try and rescue the soldiers, as well as his friends Chico, Sophie, and Professor Blaine. Unfortunately, he arrives only after Skull has escaped, taking Sophie as a hostage after massacring nearly all the natives and many of the soldiers using his psychic abilities. Luckily, Chico and Blaine survive.
Wounded by a gunshot, Skull hides in a farmhouse to recuperate, mentally controlling everyone inside. Meanwhile, Zagor, having retrieved a wooden mask that blocks Skull’s powers, follows his trail. Upon reaching the farmhouse, he is knocked out by Sophie! She, too, possesses mental powers and is in league with Skull, her father.
Sophie reveals to Zagor that, although her telepathic powers are weaker than her father’s, she is capable of subtly manipulating people without them even realizing it. She had married Quandy only to influence him and orchestrated everything including manipulating the Secret Service and military, to create the opportunity to free her father.
Professor Blaine arrives in time to save Zagor and gives him another wooden mask. With it, Zagor confronts Skull on the farmhouse roof and manages to pull him down, killing him.
As Zagor lies unconscious, Sophie prepares to kill him, but Chico and the Munsee warriors arrive. When she tries to control all their minds at once, something snaps in her brain, she collapses into a coma and is imprisoned at Hellgate as well.
In the final pages, we discover that Professor Blaine has severed Skull’s head and transported it to the Elsewhere base, where a team of scientists can study the secrets of the man with two brains…
