Frank Bigelow (Edmond O'Brien) begins his tale by walking into a Los Angeles police station and telling a detective "I want to report a murder... mine." Oddly, the police almost seem to have been expecting him and already know who he is since there has been an APB on him since yesterday.
A flashback begins with Bigelow in his hometown of Banning, California where he is an accountant and notary public. He decides to take a one-week vacation in San Francisco, but this does not sit well with Paula Gibson (Pamela Britton), his confidential secretary and girlfriend, as he does not want her to accompany him.
Bigelow meets and accompanies a group of randy men from a sales convention on a night on the town. At a "jive" nightclub called "The Fisherman", unnoticed by Bigelow, a stranger swaps his drink for another. (The nightclub scene includes one of the earliest depictions of the Beat sub-culture). The next morning, Bigelow feels ill. He visits a doctor at the general hospital, where tests reveal he swallowed a poison called "luminous toxin" for which there is no antidote. A second opinion confirms the grim diagnosis, and the other doctor implies that the poisoning must have been deliberate. Bigelow remembers his drink tasted strange. He flees from the hospital in a panic, wondering who poisoned him and why.
With just a day or two to live at most, Bigelow sets out to untangle the events behind his impending death, interrupted occasionally by phone calls from Paula, whom never finds out about the predicament that he is in. Paula provides the first clue: a man named Eugene Philips who had urgently been trying to contact Bigelow by phone for the past two days has died, purportedly a suicide. Bigelow travels to Philips' import-export company in Los Angeles, meeting a variety of suspicious characters. He first meets Miss Foster (Beverly Garland), Philips' secretary; then Mr. Halliday (William Ching), the company's comptroller, who tells him that Eugene Philips committed suicide after jumping out of a window from his apartment the previous day, but he cannot offer Bigelow any information on why Philips was attempting to contact him or why. From there, Bigelow follows a lead to Philips' widow (Lynn Baggett) (whom is strangely distraught to meet Bigelow despite the fact that they have never met before). Bigelow also meets Philips' younger brother Stanley (Henry Hart) whom also claims not to know what Philips was wanting to speak with Bigelow for.
The key to the mystery is a bill of sale for what turns out to be a shipment of stolen iridium. Stanley tells Bigelow that Philips was arrested two days before he died for selling the iridium to a gangster named Majak. During another phone talk with Paula, Bigelow learns that he had unknowingly notarized the bill of sale document of the iridium for Eugene Philips six months earlier on the behest of Philips' friend George Reynolds. Bigelow meets with Mrs. Philips again who tells him about Reynolds who had went missing two months earlier and who may have been responsible for the theft of the stolen iridium. Bigelow then goes back to Miss Foster who tells him that Philips had been having an affair with a fashion model and that Mrs. Philips knew about it.
Bigelow meets with Philips' mistress, Marla Rakubian (Laurette Luez) demanding what she knows about Philips and Reynolds as well as the sale of the iridium, but she pulls out a gun and attempts to kill him. He subdues her and takes her gun. He demands to know what she knows, but she claims not to have seen George Reynolds for months. Bigelow takes a photo she has of Reynolds and goes to a photo studio where the photo was taken where the manager and photographer claim that the photo was taken over a year ago, but that Reynolds used a different name. While leaving the studio, an unseen sniper attempts to shoot Bigelow as he gets into his rental car. After a chase through a nearby warehouse, the unseen sniper gets away, allowing Bigelow to go free.
That evening, Bigelow is captured outside his hotel by a group of gangsters whom take him to Majak (Luther Adler), a powerful crime lord whom is Marla's real lover after she had called him about Bigelow making inquires. After Majak questions Bigelow over what he knows and "what Philips may have told him over the phone". Majak sees that the ignorant Bigelow indeed did not talk at all to Philips in the past two days, but Bigelow has learned too much about Majak buying the stolen iridium from Philips so that Majak could re-sell it on the black market, plus George Reynolds was in fact murdered by Majak to cover up loose ends. Majak orders his psychotic chief henchman Chester (Neville Brand) to take Bigelow out of the city and kill him (none of the gangsters seem to be aware that Bigelow has been poisoned). However, Bigelow manages to escape when he stops the car that Chester is driving through the city and runs out. Chester pulls out a gun and gives chase, attemping to shoot Bigelow on the busy sidewalk. Bigelow runs into a pinball arcade where the police show up and shoot Chester dead, allowing Bigelow to slip away.
Bigelow thinks Stanley and Miss Foster are his killers but when he confronts them at Philips' office, he finds Stanley has been poisoned too - after having dinner with Mrs. Philips. He tells them to call an ambulance and what poison has been ingested so that (in Stanley's case at least) prompt treatment may save his life. Stanley tells Bigelow he found evidence that Halliday and Mrs. Philips were having an affair. Bigelow realizes that the theft of the iridium by Majak was merely a diversion. Philips discovered the affair and Halliday killed him.
Bigelow then confronts Mrs. Philips at her house where she tells him that she and Halliday used the investigation of the iridium as a cover for their crime, making it seem that Eugene Philips had killed himself out of shame. However, when they discovered that there was evidence of his innocence in the notarized bill of sale, Halliday murdered anyone who had knowledge of the bill of sale.
After leaving Mrs. Philips apartment building, Bigelow is jumped by several of Majak's henchmen who try to kill him. Bigelow manages to outrun them and boards a local bus, but the gangsters, led by Majak himself, follow the bus in a car. When the bus stops at a local stop, one of the henchmen attempts to board, but Bigelow disembarks and sees a policeman standing at the street corner and stands close to him. Seeing that Majak and his henchmen will not attempt to capture or kill Bigelow with the cop standing close by, they drive away, allowing Bigelow to escape again.
Bigelow tracks Halliday down to an office where he confronts him and finally sees that Halliday was the stranger who poisoned his drink the previous night. When Halliday attempts to run, Bigelow shoots him to death in an exchange of gunfire.
The flashback comes to an end. Bigelow finishes telling his story at the police station and dies, his last word being "Paula..." The police detective taking down the report instructs that his file on Bigelow's death be marked as dead on arrival, or "D.O.A."