Nick Trayne, an eccentric former private investigator who retired to pursue a more leisurely occupation, is lured back into detective work by Ed Moline, whose best friend, banker Walter Craig, has disappeared from his home. At the estate, Nick meets the Craig family, consisting of Walter's second wife Helen, his snobby daughter Tina, her fiancé, Arthur Wallace, Walter's nutty sister, Delia Phillips, and her husband George, as well as Walter's friend and former partner, Tony Weldon, and Walter's secretary, Billie Hilton, who assists Nick in his work. During Nick's interrogations, Walter suddenly reappears in the house in a cataleptic state. Brain specialist Dr. Bruhling determines that a portion of the brain cells in Walter's cerebral cortex have been paralyzed, something which can only be done on purpose. Nick questions the various family members but learns little except that if Walter dies, Tina will inherit everything. When he arranges to meet George in the garden, he finds the man knifed to death and Walter standing over the body. After a false clue points to Wallace as the killer, Walter, who according to Bruhling is capable of violent behavior, attempts to kill Nick. Using additional information provided by Bruhling, Nick tracks a piece of equipment that could have caused Walter's brain damage to a man named Carson, who has rented an abandoned house near the Craig estate. Nick and Billie, who have fallen in love, go to the abandoned house on a stormy night and, after finding nothing, are nearly shot by an unseen assailant. Just before they leave the house, they discover an inhabitant in the same condition as Walter. They then go to see Homer Hawkins, the realtor for the abandoned house, but he claims that his only contact with Carson was over the telephone. Nick devises a plan to flush out the killer by recording the voices of everyone at the Craig house, claiming that Hawkins will be able to identify Carson by his voice. Late that night, both Tony and Helen attempt to kill Nick, and he calls the police. With the guilty couple arrested, Nick explains their motives: Tony and Helen were in love and Craig found out about their affair. Knowing that Tina was in line for Craig's inheritance, Tony pretended to be a physician named Carson and purchased the machine that caused Walter's brain damage, thereby ensuring that Helen would be able to handle Walter's estate without Tina. Tony killed George because he saw Tony return Walter to the house. Nick is thrilled when Tina offers to pay him $25,000 for his work, and he and Billie run off to get married.