It's a hot day (like pretty much every day) in Logan City as Lois Reed (Carol Kelly) arrives on the bus and is met by private investigator Donald Briggs (Harry Landers). He has hired her just to show up in the town, and takes her to her room at the Sands Motel, next door to his own room. Later, Briggs visits Wilma Adams (Olive Blakeney) , who was also summoned to Logan City and is staying at a boarding house. He's there because Wilma's son Marv (Gary Vinson), a chemical engineering student, is engaged to Helen Waters (Carolyn Craig), daughter of local big-shot Clyde Waters (Victor Sutherland). Briggs says he knows part of a secret about Marv's father, and will uncover the rest of it thanks to the presence of Lois Reed. He says that $5000 will make him less curious. Marv arrives, having learned that his mother is in town. Briggs leaves, but Marv quickly sees that the P.I. has upset Wilma, so he follows Briggs outside and they get into a fight. It's broken up by Police Chief Glass (Rusty Lane).
Briggs next reports to Clyde, who initially hired him to look into the mystery surrounding his prospective son-in-law's father. Briggs tells him that Marv's father was actually Ben Devereux, a local man who was executed for murder. Clyde pays Briggs for his services, but Briggs wants another $5000 to keep the matter quiet, and again mentions that Lois Reed is in town. Later, Clyde visits Perry in L.A. and explains that he has nothing against Marv, but merely wondered why the boy seemed to know so little about his father. With Briggs intent on blackmail, it seems matters have spun out of control, so he needs help from Perry and Paul. He adds that Devereux was convicted of murdering his partner 18 years earlier. Perry will look into the trial record, while Paul looks up Briggs.
In Logan City, Perry returns the transcript of the Devereux trial that he has borrowed from Judge Meeham (Nolan Leary), who was the prosecutor in that case. Perry questions why both Meeham and the defense attorney permitted a "Miss X" who was mention several times in the case, was allowed to remain anonymous. The judge replies that she wasn't involved in the murder or its motive, so it was agreed not to have her suffer embarrassment. Perry is at a filling station intending to return to L.A. when Chief Glass drives up and insists that he follow him to the Norris ranch. There, meets retiree George Norris (Don Beddoe) and his wife Martha (Paula Winslowe). She says that the man Devereux killed was her first husband, David Latwell, and she doesn't want all this mess dug up again.
Perry next goes to the Waters ranch, where he, Clyde, and Helen observe Marv entertaining some schoolchildren. He pours some "magic water" on a duck and sets the bird in a water trough. After a moment, it sinks, and one of the children quickly rescues it. Marv reveals to the adults that what he poured on the duck was a chemical that removes oil from the duck's feathers. Perry gets a call from Paul, who has tracked Briggs to Logan City, and they agree to visit Briggs that evening. When they arrive at the Sands Motel, they encounter Lois, who heard them drive up and thought it was her cab. She says that she heard someone visiting Briggs half an hour earlier and believe they may have left together. Once she returns to her room, Perry and Paul enter Briggs' unlatched door. They find some old newspaper clippings and notice a strange odor. Then they spot the man himself - dead.
Later, as the police investigate, Dr. Creel (Joseph Forte) says that Briggs was killed with poison gas - potassium cyanide pellets tossed into a container of hydrochloric acid would do the trick. The Chief, adding this bit of chemistry to Marv's earlier fight with Briggs, plus Lois having spotted Wilma's truck (which her son had been borrowing), decides that the young man is his chief suspect. He heads to the Waters ranch, where Marv had been staying, but Perry knows that Marv was about to return to school. He and Paul drive to the bus stop and find Marv there. Perry drives off with him, and hears from Marv that he never saw Briggs again after their brief fight. Chief Glass pulls them over, arrests Marv, and admonishes Perry for his interference.
In jail, Perry can't get Marv to take interest in his own defense. He suggests that Marv is afraid the proving his own innocence will shift blame for the murder to his mother. After that, Marv won't talk to Perry any more. Perry gets a call from Lois, who tells him to ask Judge Meeham who really defended Devereux. He does just that, and learns that while another man handled the case in court, the defense was prepared by George Norris. He's the one who agreed with Meeham to leave "Miss X" unidentified.
Perry calls on George, who first explains that what is now the Norris ranch was at the time of the murder co-owned by Devereux and Latwell. However, they argued constantly. Perry asks if George agreed to leave Miss X anonymous to sabotage his own case, but George admits that that was merely the result of his shortcomings as a lawyer. He still doesn't know if Devereux was guilty or innocent. The motive for protecting Miss X was that her father was the beloved Reverend Logan, for whom the town was later named. However, Miss X left town at the time of the trial and didn't even return for either her father's funeral or the town renaming ceremony. She's been married and divorced, and is now Lois Reed. Perry visits Lois, who tells him that Briggs offered her $1000 just to show up in town for a couple days. About two hours before his body was found, he visited her and she told him she quit. He wrote down three names on a piece of paper and circled one, calling it the "murdering goose who laid a golden egg". Lois never got a look at what he wrote.
In court, pompous D.A. Harry Cortland (Philip Tonge) is having Helen testify to Marv's knowledge of chemical tricks. When Clyde testifies to the "drowning duck" trick, Cortland suggests that Marv might have let the duck drown if the boy hadn't intervened, but Perry objects and Judge Meeham sustains him. During a recess, Paul gives Perry a report on Wilma's bad heart - it's surprising that she's lived even as long as she has. Perry uses this fact to get Marv to cooperate at least to the extent of telling his attorney where Wilma has been staying.
Paul and Della go there, but find her dead (from natural causes). Next to the body is a small strongbox, which Paul and Perry later open with Marv's permission. Inside are various papers that Wilma had collected from Ben's desk, with photos of Lois and an incomplete letter that basically says "My Dear, We both know our marriage is over. I love someone else. We can discuss a settlement after I return from my weekend trip." Perry visits Lois, who's a bit drunk. She laughs at the idea that she'd marry the short, dumpy, square, and bald Ben Devereux. Her affair was with Latwell, who shared a desk with his partner. 18 years late, she sounds like she's still in love with him.
The next day, Lois testifies in court about how Briggs brought her to Logan City in what she finally learned was a blackmail scheme against multiple people, but she doesn't know whom. Dr. Creel testifies about poisoning with HCl and KCN. He says that these are commonly found around a farm for pest control, and that using them would not require a chemist's special knowledge. Paul enters with a copy of a newspaper clipping from Briggs' room at the Sands. Perry calls Martha Norris to the stand. She repeats her testimony from the Devereux trial: When her husband was murdered, she was laid up with a self-treated sprained ankle from a tractor accident. When she claims she and Latwell were in love, Perry introduces the incomplete letter, which was never shown in the earlier trial because it would have required calling "Miss X" to explain its significance. Perry suggests that Briggs was murdered because he discovered that Martha killed her husband and intended to blackmail her. She repeats her ankle alibi, but Perry introduces the newspaper clipping into evidence. It's dated the day of the murder and has a photo of Lois and Latwell at the county fair, with Martha standing in the background. Martha breaks down, but says that her husband was better off dead than with someone like Lois.
Later, Marv complains that the town refused to believe his father. Perry sets he shouldn't let his resentment color the rest of his life. Marv asks how it could not, when the whole town also turned against him. Perry reminds him that Helen is part of the town. The couple look into each other's eyes, just smiling, until Della says "Don't just stand there. Do something!" As they kiss, Della looks satisfied with herself, while Perry and Paul examine some papers.