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New York detective Duncan Maclain refuses to allow blindness to interfere with his work, which is aided by his faithful dog Friday, his butler, Alistair, and his assistant, Marty. One day, actress Norma Lawry, an old friend, visits Mac and asks him to help her prevent her stepdaughter Barbara from doing something foolish with actor Paul Gerente. Paul, a cad who had once been Norma's lover, has convinced Barbara that Norma only married her father Stephen for his money. Mac tells Norma that she doesn't need a "gumshoe" and suggests that she talk to Paul herself. Paul refuses to listen to Norma's pleas and insists that he loves Barbara, who is appearing in a local theatrical production with him, even though she is only seventeen. That night, Stephen has to go away on a business trip to test a secret formula on which he has been working for the government. Because Norma is worried about Barbara, she decides not to accompany him and goes to Paul's 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, who hates Norma, 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, but finds the body missing. While he is searching for clues, a man arrives, telephones Norma's number and talks to someone. Mac and Friday are able to overcome the man by extinguishing the lights, and although Mac learns the man's identity, Gabriel Hoffman, Gabriel reveals nothing more and Mac has Marty hide him. Mac then goes to Norma's house in the country and pretends to be an uncle who has come for a visit. Shortly thereafter, Barbara comes home from her rehearsal, accompanied by her director, Cheli Scott, whom she has invited to spend the night. Unknown to Barbara and Norma, Cheli is a Nazi agent who is working with Hansen, the Lawry's butler, to steal the plans to Stephen's invention. Cheli ordered Paul killed because his affection for Barbara was beginning to limit his effectiveness as a spy. Gabriel and his wife Vera, who is a maid at the house, are also working with Cheli, and when Gabriel does not return home after his "assignment," Vera becomes hysterical with worry. Mac, whose pretense of being a demanding old curmudgeon has distracted Hansen, secretly asks Vera to meet him in the greenhouse. She is observed by her cohorts, however, and is killed before she can reach Mac. The spies then cut the house's telephone wires so that Mac and Norma cannot reach the police, but Mac slips a note into Friday's collar and tells him to "go home." While Friday runs to Mac's, Stephen arrives home, and Cheli threatens his family if he does not give her the entire formula. Mac tries to stall for time, but is taken to the cellar, where his advantage of having "eyes" in the dark, helps him to overcome his captor. Just then the police and Marty arrive, having been summoned by Friday's note. Finally, Barbara realizes how much Norma cares for her, and the Lawrys become a happy family.