Private detective Duncan MacLain is blind but has developed all his other senses. He works with his loyal guide dog Friday, his butler Alistair, and his assistant Marty to pursue his work. Retired actress Norma Lawry, an old friend, visits Mac for advice about her 17-year-old daughter Barbara who is enamored with her leading man, Lothario Paul Gerente in a small theater production.
Norma tries to reason with Paul, who insists 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 on a trip to test a secret invention for the government and the war effort.
When Barbara arrives, she finds Paul dead and thinks Norma killed him. She threatens to call the police unless Norma leaves. Norma agrees but 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. Marty hides him to keep him under wraps. Norma returns 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 at Norma's house pretending to be her curmudgeonly "Uncle Mac" who has come for a visit. Shortly thereafter, Barbara comes home from an "emergency rehearsal" called 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.
Using her worry about Gabriel as a lure, Mac secretly asks Vera to meet him in the greenhouse. She is observed by others, however, and is killed by Hansen before she can reach Mac. The spy cut the house's telephone wires so that Mac and Norma cannot call the police. Cheli has Mac locked up under guard.
Mac uses his blindness as a distraction and 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 breaks 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."
