The film was followed by the sequel (also starring Arnold) The Hidden Eye.
Plot
New York private detective Duncan "Mac" MacLain is blind but has keenly developed all his other senses. Skilled in self-defense, he pursues his work with the help of his faithful guide dog Friday and assistant Marty.
Retired actress Norma Lawry, an old friend, comes seeking advice. Her headstrong 17-year-old step-daughter Barbara, herself a promising actress, is enamored with her leading man, the much older Lothario Paul Gerente. Paul, who once had been Norma's lover, has convinced Barbara that Norma actually wants him for herself and only has married her father Stephen for his money. At Mac's urging, Norma tries to reason with Paul, but he insists that he loves Barbara and laughs her off. Barbara is equally disdainful of Norma, and to spite her calls Paul in front of her to set up a tryst with him that night at his apartment.
Norma's husband Stephen is a scientist leaving on a trip to test a secret invention he has been developing for the U.S. WW II war effort. Worried about Barbara, she uses a pretext not to accompany him. When Barbara gets to Paul’s apartment she finds him dead and Norma there, and concludes that Norma has killed him; Norma insists she'd found him that way. Barbara threatens to call the police unless she leaves Stephen.
Fearing that a scandal will hurt Stephen's efforts, Norma agrees to Barbara’s blackmail. 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 overhears him call to Norma's number saying it's "Gabriel" and asking for "Vera." Mac accidentally gives himself away, but with Friday's help overpowers the thug, who refuses to talk. Mac has Marty hide him to keep him under wraps.
Norma is out when Mac arrives at her house pretending to be her curmudgeonly "Uncle Mac" who has come for a visit. His blindness lulls any suspicions Hansen the butler - second in command of a German espionage ring seeking to get Steven’s secret plans - might have about him. Hansen had Gabriel kill Paul, whose affection for Barbara was beginning to limit his effectiveness.
When Gabriel fails to return home his wife Vera, the Lawrys' maid, becomes hysterical with worry. Mac secretly asks her to meet him in the greenhouse. She is observed by the others, however, and is killed by Hansen before she can reach the detective.
Norma returns home, which interrupts the ring’s effort to break into Stephen's safe. Shortly thereafter, Barbara returns from an "emergency rehearsal" called to get her out of the house by her director, Cheli Scott, the spies' ringleader. She inveigled an invitation to spend the night at the Lawry home, and immediately suspects Mac is not what he seems.
The spies cut the house's telephone wires, and Cheli has Mac placed under guard. Using his blindness as a distraction, he overpowers his guard and sends Friday to fetch Marty.
Stephen arrives home and Cheli threatens to have Norma’s face disfigured if he does not surrender the final step to his invention. Barbara finally realizes Norma had not killed Paul, and how much her father cares for his wife. Mac tries to stall for time, but is locked in the cellar. When Hansen is sent to kill him, Mac knocks out the light there to gain an advantage in the total darkness, then overpowers him.
Marty and Friday return with the police. After the ring is captured, Stephen and Norma are off to Washington, D.C.. Barbara, however, remains cheerfully behind to paint the town with her new "Uncle Mac."
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