Frank Bigelow, told he's been poisoned and has only a few days to live, tries to find out who killed him and why.
Small-town accountant Frank Bigelow goes to San Francisco for a week's fun prior to settling down with fiancée Paula. After a night on the town, he wakes up with more than just a hangover; doctors tell him he's been given a "luminous toxin" with no antidote and has, at most, a week to live! Not knowing who did it or why, Bigelow embarks on a frantic odyssey to find his own murderer.
Frank Bigelow goes off to San Francisco for a week's holiday. He's a self-employed accountant and is engaged to his assistant Paula Gibson. He checks into the St. Francis Hotel and is soon partying with a group of salesman who are having a party. He ends up at a jive bar where a mysterious stranger switches drinks with him. He awakens the next morning feeling unwell and after having some medical tests is told that he's been poisoned and there is no cure. He has one week to live. When Paula tells him that a man in Los Angeles had been trying to reach him, he sets off to learn if there might be any connection to his current situation. Unfortunately - and too conveniently - the man committed suicide by jumping off his high rise apartment balcony not long after trying to contact him. He soon learns that some very shady characters are involved but why they might be interested in him is what he really wants to know.
In Los Angeles, the accountant Frank Bigelow goes to the Homicide Division of the Police Station to report his murder and the captain and detectives listen to his intriguing story. Frank has an accounting office in the small town Banning, on the way to Palm Springs. He tells his secretary and fiancée Paula Gibson that he will travel alone to San Francisco to spend a couple of days on vacation expecting to have fun. Frank lodges in a hotel and befriends a group that is participating in the market week event. They go to The Fishermen and Frank is bothered by Sue, who is married with his host. Frank leaves the woman and meets the lonely socialite Jeanie and a man swaps his bourbon with another. Meanwhile Paula calls him telling that a man called Philips from Los Angeles wants to meet him. The next morning, Franks wakes up with hangover and goes to a medical building and the doctors tell him that he had drunk luminous toxin poison that affects vital organs and there is no antidote. Frank rushes to the Southern Pacific Hospital and the doctors confirm the diagnosis and give one week of life maximum. Frank Bigelow decides to investigate the reason why he was poisoned in a quest to find the truth.
- 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 two days earlier 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 arrives at his hotel, Paula telephones to tell him that Eugene Philips, of Philips' Import and Exporting Company, urgently needs to speak with him and refuses to leave a message. That evening Sam Haskell, a guest in the room opposite Bigelow's, invites him to a party in his suite. Bigelow accompanies Haskell and his friends, 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. He first returns to the Embarcadero bar, which is closed, then looks for Haskell, who has already checked out of the hotel. During a phone call with Paula, she inadvertently provides the first clue: the unknown businessman Eugene Philips, who had urgently been trying to contact Bigelow by phone for the past two days has suddenly died, purportedly a suicide. Driven by an impulse, 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 high-rise 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. However, Stanley does explain that his brother had sold a rare form of iridium, a luminous toxin, and faced possible imprisonment, which may have drove him to suicide.
Back at the hotel, Paula again telephones Bigelow to say she has discovered that some months prior, Bigelow notarized a bill of sale of iridium from a man named George Reynolds who was working in league with Philips. Returning to question Mrs. Philips, Bigelow discovers that both the bill of sale and Reynolds have disappeared. Believing that all evidence of the sale is being systematically eliminated, Bigelow thinks he has the motive for his poisoning and is determined to find the culprit. Convinced that Philips was also murdered, Bigelow again questions Miss Foster, who reveals that on the day of his death Philips saw Marla Rakubian (Laurette Luez), a model and former girl friend.
Bigelow goes to see Marla and accuses her of being in league with Reynolds. When she pulls a gun on him, he disarms her and then takes both the gun and a portrait picture she has of Reynolds. Bigelow tries to locate Reynolds through the portrait studio, but instead discovers Reynolds' real name is Raymond Rakubian. When he goes to see Reynolds at the address provided by the photographers, Frank finds an abandoned warehouse and is fired upon by an unseen sniper from a warehouse. Bigelow chases the sniper, who gets away.
That evening, after returning to Philips' office, Bigelow is kidnapped by Chester (Nevile Brand), a psychotic henchman of Reynolds' gangster uncle, Mr. Majak, the illegal purchaser of the iridium. He is taken to Majak himself, where his current mistress, Marla, is beside him. Majak insists that his nephew cannot be connected to the murders as he has been dead for the past five months.
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, attempting to shoot Bigelow on the busy sidewalk. Bigelow runs into a soda fountain pharmacy 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."