I, The Jury recap

Private investigator Mike Hammer finds his wartime friend Jack Williams murdered and vows to execute the killer personally. Working uneasily beside homicide detective Pat Chambers, he follows Williams's contacts into a network involving narcotics, vice, vulnerable young people, and psychiatrist Charlotte Manning. Further violence removes witnesses and complicates the distinction between the central murder and the criminal operations surrounding it. Hammer uses his secretary Velda, police information, intimidation, and armed confrontations to eliminate suspects. At the same time, he becomes romantically involved with Charlotte, whose poise and apparent helpfulness place her above his suspicion. The accumulated evidence finally shows that Charlotte killed Jack and concealed her role behind the other crimes. Hammer confronts her alone. When she tries to exploit his desire for her, he shoots her in the abdomen, fulfilling his promise to Jack instead of making an arrest. The case closes with Hammer acting as the jury and carrying out his own sentence.

Private investigator Mike Hammer is called to the apartment of Jack Williams, his closest friend and former wartime comrade. Williams has been shot in the abdomen and left to die slowly. Homicide detective Pat Chambers controls the official scene, but Hammer immediately makes the crime personal: because Jack once saved his life, he promises the dead man that he will find and kill the murderer. The shot, the room, and the people known to have contact with Williams provide the first leads. Hammer begins questioning those contacts while his secretary Velda helps organize the search. Pat warns Hammer that solving a murder does not authorize an execution, but the warning does not change his stated purpose.

Hammer and Pat Chambers pursue overlapping versions of the investigation. With help from his secretary Velda, Hammer traces Williams's concern for people caught in narcotics and vice, including Myrna Devlin, and looks for the criminal interests that might have wanted him silent. George Kalecki's underworld connections make him a natural suspect, while Hal Kines and other members of Charlotte Manning's circle provide a second set of personal relationships to examine. Charlotte offers insight into the people involved and grows closer to Hammer. He is willing to threaten or fight men who block him, but the expanding list of motives gives him no single proven killer. The case now connects Jack's private loyalties, illicit business, and the lives of Charlotte's patients and acquaintances.

Further attacks and deaths show that the murderer is still protecting a secret and may be monitoring Hammer's progress. He tests alibis, compares the victims' relationships, and pushes into the narcotics organization suggested by Williams's contacts. Some dangerous figures are guilty of other crimes but do not fit the physical and chronological facts of Jack's shooting. Pat continues to supply official findings while trying to keep Hammer from turning every confrontation into a killing. Charlotte remains apparently helpful and becomes Hammer's principal emotional refuge during the investigation. That intimacy makes her part of his private future even as her access to the people in the case gives her unusual knowledge of its details.

Hammer's pressure breaks open the surrounding criminal activity, but eliminating its obvious enforcers does not fully explain Jack Williams's murder. The sequence of later crimes indicates a killer who can move among respectable people as well as the underworld and who understands how the victims are connected. Hammer revisits statements that initially seemed harmless and notices where knowledge, opportunity, and motive overlap. His relationship with Charlotte Manning deepens, creating a conflict between the evidence and the person he wants to trust. By narrowing the timeline and separating the vice operation from the hand that shot Jack, he approaches a conclusion that the official investigation has not yet acted upon.

Hammer concludes that Charlotte Manning killed Jack Williams and used her respected position, her knowledge of the people around him, and the surrounding criminal network to deflect suspicion. The later violence was part of protecting that concealment. Instead of bringing Pat Chambers to make an arrest, Hammer confronts Charlotte alone and explains what the clues establish. Charlotte attempts to exploit his love and physical desire, expecting their intimacy to overcome his accusation. Hammer remains committed to the vow he made over Jack's body. He shoots her in the abdomen, inflicting the same kind of death as Jack's, and answers her disbelief without remorse. The murderer is identified and killed, but there is no trial: Hammer has chosen to serve as investigator, jury, and executioner.

Mike Hammer determines that Charlotte Manning, the woman he has come to love, killed Jack Williams and manipulated the later violence around the investigation. Her medical standing and composed assistance had allowed her to remain close to the inquiry while concealing her own criminal connections. Hammer confronts her privately and lays out the case rather than turning immediately to Pat Chambers. Charlotte attempts to use Hammer's feelings and attraction to disarm his resolve, but he follows through on the promise made beside Jack's body. He shoots her in the abdomen, deliberately mirroring the slow death inflicted on his friend. Charlotte asks how he could do it; Hammer answers that it was easy. The murder is solved, but the ending makes clear that Hammer has acted as self-appointed judge and executioner rather than delivering the killer to legal trial.