Shortly before Christmas in New York City, one-armed insurance investigator Jack Williams, Robert Swanger, is staring at a college yearbook photo of John Hansen when someone slips into his apartment and shoots him to death. Hot-headed private investigator Mike Hammer, Biff Elliot, Jack's war buddy, vows to avenge his friend's death despite a warning from Pat Chambers, Preston Foster, captain of the homicide squad, to let the police handle the case. Pat is unable to calm Mike, who roughs up a wisecracking reporter before leaving the crime scene. Knowing that Mike will forge ahead with an investigation regardless of his advice, Pat urges the offended reporter to publish an article disclosing that Mike is on the job.
Mike heads to see Jack's fiancée, Myrna Devlin, Frances Osborne a torch singer and reformed drug addict, but she is too distraught to talk. The next day, Mike's secretary Velda, Margaret Sheridan tells him about the article, titled “I, the Jury,” which suggests that Mike knows the killer’s identity and thus becomes a target. Because Pat has issued a guest list from Jack's recent party, Mike suspects the police captain is using him as bait to draw out the culprit.
The investigation leads Mike first to upstate New York to question George Kalecki, Alan Reed, a wealthy fight promoter and art collector who houses his live‑in friend, Hal Kines, Bob Cunningham, who Kalecki claims is a college student, Hal Kines. Kalecki asserts they were home after the party, but Mike glimpses a tense argument through a window as he departs. He then visits Charlotte Manning, Peggie Castle, a skilled psychoanalyst and author who treated both Jack and Myrna. Charlotte flirts with him yet gives no solid information, leaving Mike with more questions than answers.
Pat eventually catches up with him, and Mike is told that Kines moved out and believes Mike tried to shoot him. Kines’s new address sits in the same building as two party guests, Esther and Mary Bellamy. Mike searches Kines’s apartment and uncovers photos of Kines and Kalecki in Europe before and after World War II. When Kines returns unexpectedly and grabs Mike’s arm, the detective beats him, then climbs upstairs to talk with Mary, who knew Jack when he guarded her father’s estate. Mary resists Mike’s interrogation but confirms that Charlotte drove her, Myrna, and Esther home after Jack and Myrna argued that night.
Back at his office, ex‑boxer Killer Thompson reveals to Mike and Velda that Kalecki, his former manager, runs a numbers racket. Mike pursues further leads about the racket but earns a brutal beating from Kalecki’s people. Charlotte tends his wounds and, with a kiss, lifts his spirits, then asks whether Jack left a message for Mike before he died.
Mike sneaks into Jack’s apartment through a window to avoid the guard, and finds a note from Pat, who anticipated his arrival. He also discovers Jack’s diary, which mentions a woman named Eileen Vickers, Mary Anderson, who had changed her name to Mary Wright, along with a note that Jack planned to raid a dance school with the police in a few days. Mike locates Eileen at a dance school that serves as a front for prostitution and learns she’s shocked by Jack’s death but only knows he wanted her to seek help from Charlotte.
With so many threads, Mike and Pat expand their search to yearbooks and soon identify Hal Kines as John Hansen, the man Kalecki claimed was a student. After police raids on the dance school, they discover dead bodies—Eileen and Kines—in Eileen’s room. Kalecki confesses that he argued with Kines about Esther’s involvement, but Mike remains puzzled as to why Kines, who had posed as a student for years, would be connected to the murders. The tension escalates when Charlotte and Mike are nearly killed by a shot fired outside his office. That night, Mike is warned by Bobo, Elisha Cook Jr., a slow-witted former boxer now working as a department-store Santa, who cryptically warns that “the big man” is after him.
Velda suspects that Kines ran Kalecki’s jewelry scheme at the college, using his student identity as cover. A search of Kines’s room at the fraternity house reveals Kalecki inside the burning room, destroying Kines’s papers. Kalecki fires at Mike and is killed when Mike shoots back, grabbing Kalecki’s gun just as the police arrive to arrest Kalecki.
Angered by Mike’s unilateral justice, Pat has him released from jail the next day and, with Kalecki’s gun in hand, they examine Kalecki’s safe-deposit box, which holds a cache of stolen vintage European jewelry. The detectives conclude that Jack had indeed been investigating Kalecki and Kines, who had been fencing European loot for years, and that all four murders were tied to the same weapon—yet not Kalecki’s gun. Pat deduces that Myrna, once a jewel thief, may have been influenced by Kalecki to eliminate Jack.
As Mike confides in Charlotte, with whom he has fallen in love, he theorizes that Kines recruited campus thieves for Kalecki. Pat learns that Myrna is drunk in a bar and sends Mike and Charlotte to bring her to Charlotte’s apartment to sober up. After Mike leaves, Charlotte administers sodium pentothal to Myrna, trying to extract information about Jack, but Myrna remains unclear. Meanwhile, Mike is beaten by Kalecki’s thugs at his office, but he turns the tables and the assailants are arrested, offering little new information.
When Myrna is found dead in the street from a hit-and-run, the medical examiner notes a needle mark on her arm, prompting Pat to conclude she had fallen back into drug use. Mike realizes that Charlotte murdered Myrna and suspects that Charlotte, having learned of the jewelry racket during a hypnosis session with Kines, plans to take over Kalecki’s business. Mike waits for Charlotte in her apartment and directly accuses her. Charlotte pretends to seduce him, but, reaching for a hidden gun, she embraces him—and Mike shoots and kills her.
