New York private detective Duncan Mac MacLain is blind but has keenly developed all his other senses       He pursues his work assisted by his faithful guide dog Friday, his butler, Alistair, and his assistant, Marty       Retired actress Norma Lawry, an old friend, comes to Mac seeking advice       Her headstrong 17-year-old step-daughter Barbara, herself a promising actress, is enamored with her leading man in a small theater production, the much older Lothario Paul Gerente       Paul, who had once been Norma's lover, has convinced Barbara that Norma actually wants Paul for herself and has only married her father Stephen for his money       At Mac's urging, Norma tried to reason with Paul, but he insists that he loves Barbara and laughs her off       Barbara is equally disdainful of her and agrees to meet Paul for dinner that night at his apartment      　　Norma's husband Stephen Lawry is a scientist leaving that night on a trip to test a secret invention on which he has been working for the government and the war effort       Worried about Barbara, she uses a pretext not to accompany Stephen and goes to the apartment       When Barbara arrives, she finds Paul's dead body and thinks that Norma has killed him, even though Norma insists that he was dead when she arrived       Barbara threatens to call the police unless she leaves Stephen       Fearing that a scandal will hurt him, Norma agrees       Norma then turns to Mac, who goes to Paul's apartment with Friday and Marty, only to find the body and the rug under it missing       A man arrives to replace the rug and Mac hides in a closet, overhearing a telephone call to Norma's number from Gabriel to Vera       Mac accidentally gives himself away, but with Friday's help overpowers Gabriel, who refuses to talk       Mac has Marty hide him to keep him under wraps      　　Norma returns to her country home unexpectedly, which discourages an espionage ring that includes her butler, Hansen, from breaking into Stephen's safe and stealing his plans       They have killed Gerente, who was also an agent, and hidden the body       Mac arrives with Friday at Norma's house and pretends to be her curmudgeonly Uncle Mac who has come for a visit       His blindness lulls any suspicions Hansen might have about him       Shortly thereafter, Barbara comes home from an emergency rehearsal, called to get her out of the house by her director, Cheli Scott, who inveigles an invitation to spend the night at the Lawry home       Unknown to Barbara and Norma, Cheli is the ringleader of the spies       Cheli ordered Paul killed because his affection for Barbara was beginning to limit his effectiveness, and she immediately suspects Mac is not what he seems      　　Gabriel's wife Vera, the Lawrys' maid, is also a part of the plot       When Gabriel does not return home, Vera becomes hysterical with worry       Mac secretly asks Vera to meet him in the greenhouse, using her worry about Gabriel as a lure       She is observed by the others, however, and is killed by Hansen before she can reach Mac       The spies cut the house's telephone wires so that Mac and Norma cannot call the police, and Cheli has Mac locked up under guard       Using his blindness as a distraction, Mac overpowers his guard and releases Friday out a window with instructions to go home and get Marty       Stephen arrives home and Cheli threatens his family if he does not give her the final step in creating his invention, which was not in the safe       Barbara finally realizes how much Norma actually cares for her       Mac tries to stall for time, but is locked in the cellar       When Hansen goes to kill him, Mac has broken the light bulb and his advantage of total darkness helps him overcome the spy       Marty and Friday return with the police       When Friday is spotted by a lookout, Mac overpowers him and is saved from being shot by another lookout by Friday       After the spies are captured, Stephen and Norma are off to Washington, D      C      , with Barbara remaining behind to care for her new Uncle Mac