Trama

Diana Reynolds arrives at Mason's office wearing a black eye concerned about her boss' grandson Bobby Carter. Marian Shaw, Diana's former roommate, also concerned with the grandson, is murdered at the Carter residence and Diana is charged.

When Marian Shaw returns to her apartment she tells her roommate Diana Reynolds to move out. When Diana refuses, Marian pulls a gun which goes off during their fight. Later, Diana, now sporting a black eye, hires Perry Mason after being accused of theft and losing her job. Diana was working for the wealthy Bartlett family and was involved with patriarch Mathew Bartlett's stepson, Tony Davis. Tony's mother Helen Bartlett fired Diana after (she claims) finding some of her jewelry among Diana's things. Diana had recently become aware that her roommate Marian was carrying on with Tony as well, but it was Tony who gave her the black eye. To complicate matters further, the elder Bartlett has only recently become aware that he has a grandson and will seemingly do anything to get custody. Tony discovered and stole a letter about the grandson from Diana's desk. When Marian Shaw is murdered, Diana is charged and Perry defends her.—garykmcd

Marian Shaw (General Hospital (1963)'s Rachel Ames) yells at her roommate Diana Reynolds (Hazel (1961)'s Whitney Blake) to leave, and pulls a gun on her. Diana grabs the gun, but in the struggle it goes off, sending a bullet into the wall and bringing in building superintendent Otto Kessler (Ludwig Stössel). Diana only stays at the apartment on her days off from her secretarial job at the home of wealthy Matthew Bartlett (R.G. Armstrong), where she also has a room. On her next day at work, Matthew's stepson Tony Davis (Jan Merlin) makes a pass at Diana, annoying her. He's the cause of the friction between her and Marian, whom he's seeing. It amuses the smug Tony to tell Marian that Diana keeps throwing herself at him. Diana leaves the room, giving Tony a chance to snoop. He finds an anonymous letter to Matthew claiming that his late son Robert and Robert's wife Norma Carter (Adventures of Superman (1952)'s Phyllis Coates) had a son, Bobby (Casey Peters), and that Norma is an unfit mother to Matthew's grandson. Later, Tony shows the letter to his mother, now Helen Bartlett (Irene Hervey), who only married Matthew to provide security for ne'er-do-well Tony. Helen knows the claim in the letter is plausible, so she sends Tony to visit Bobby and his mom at her house. He's useless as a spy, and Norma tells him to get out.

Helen and Tony decide to get rid of Diana by planting "stolen" jewelry in Diana's room at the Bartlett house shortly before Matthew returns from a trip. Diana says she intends to tell Matthew about the stolen anonymous letter, so they forcibly throw her out. In the process, Tony strikes her, giving her the titular black eye. Diana goes to Perry, who has Della take a photo of Diana's injury, then they drive her to her apartment. The next day, Perry goes to see Matthew, who doesn't need any convincing that Diana is the injured party. He regrets his marriage to Helen, which was the result of an emotional tailspin after the death of his son. He already knew about Bobby, as the anonymous letter writer had previously called him with the same information, which Matthew pretended to disbelieve. The letter was obviously a second attempt by the same source. Unexpectedly, Lt. Tragg enters and says that Diana has been murdered, her body and car having been found outside Norma's house. However, when they arrive at the scene, Matthew recognizes the body as Marian's. When Norma returns home, she says that she and Marian were good friends.

Diana is missing, but before long comes to Perry's office and explains: That night, she received a call that her car had been involved in an accident. This lures Diana to the scene, leaving footprints for the police to later identify, of course. She finds no one home, but then discovers Marian's body. Panicking, she finds her car won't start, so she hitches a ride to Union Station and spends the night there. Tragg arrives to arrest her as a suspected murderer. Perry interviews Diana in jail, and from her account of how she got her job, gets her to realize that Matthew only hired her as a pipeline to information about Bobby, who often visited his mother's friend Marian.

In court, D.A. Burger brings out the footprint evidence and Kessler's misleading account of the gun incident. Tony testifies that he and Marian were engaged, and that Diana was angry about it. Perry cross-examines him aggressively, bringing up the black eye. After court is adjourned, Matthew meets with Norma, begs her to let him bring up Bobby, and offers her $100,000 to make up for his earlier shabby treatment of her. At first, Norma rejects the money, but takes it when Matthew asks her to do it for Bobby's sake. Back in court, Burger introduces Marian's diary, which the police found in her apartment, to show the tension between her and Diana over Tony. Perry insists that he has the right to look over the diary, to check whether the parts that Burger had read were taken out of context. The judge lets him do so during an adjournment.

Soon, Perry and Della make a call on Norma. There were entries in the diary that meant nothing to the police or Burger, who didn't know about the situation with Bobby, but revealed much to Perry. He tells Norma he knows that she is not Bobby's mother, Marian was. (The father isn't named, but he wasn't Robert Bartlett.) Norma took the child from her unwed friend, supposedly out of kindness, but actually with the long-range goal of selling him to Matthew. She even was the source of the anonymous messages, and called herself an unfit mother to plant the seed in Matthew's mind that he should take Bobby. However, Marian's return to L.A. complicated things, so Norma murdered her to keep her from interfering. Norma leaves her house and drives off, with the police in pursuit. She swerves to avoid a bus and crashes. Dying in the hospital, she confesses. Burger moves to have Tony arrested for perjury.

In the epilogue, Perry decides not to reveal to Matthew that Bobby is unrelated to him. He does point out that raising a small boy may be a job for more than just a grandfather. Matthew takes the hint and decides to call Diana to see if she'd like to help out. Helen is apparently out of the picture. Della wonders whether Perry is doing the right thing, but when he asks her if she can think of one good reason for spilling the beans about Bobby, she admits that she can't even think of a bad reason.