On Saturday April 9, 1954, Miller Starkey, a small-time hood who works for West Coast mob second-in-command Max Edward Troy, is palling with Troy in a field when he sees hitman Chester Davitt appear over a nearby hill, sawed-off shotgun in hand. Starkey turns but is pushed by Troy and is promptly shot by Davitt.
Sgt. Joe Friday, working out of the LAPD's Intelligence Division, gets information on the killing from forensic scientist Ray Pinker as Pinker briefs division chief James Hamilton and other officers. Hamilton suspects Troy and several of his close associates, such as Lee Reinhardt, Wesley Cannon, and Davitt, and after a meeting with chief of police W. H. Parker the detectives arrest Troy and his associates and a low floor of the Crown Regent Hotel is appropriated by the police to allow mass-interrogation. Davitt cannot be found but his wife Belle is brought in and angrilly calls out Friday and company, to no avail.
Friday and his partner Frank Smith interrogate Max Troy, but after four hours they get nothing out of him. As he is taken to jail they get word that an eyewitness has been found. It is a museum curator named Jesse Quinn, who positively identifies Chester Davitt as being near the field at the time of the crime.
Friday and Smith check out The Red Spot Grille, a restaurant secretly owned by Troy, and find nothing. They then decide to infiltrate a policewoman, Grace Downing, into the bar with a primitive wire to record conversation between Troy and his pals. After 36 hours habeus corpus kicks in and despite the reluctance of Deputy DA Adolph Alexander, Troy and his pals are released. Friday and Smith, however, get information from Roy Cleaver, an informant and jazz musician; he tells them that Starkey was seen weeks before his death sporting heavy bandaging from a beating. When Friday and Smith question Starkey's widow, she tearfully confirms this and gives Friday a copy of her husband's work book, a collection of gambling debtors.
Friday and Smith question 22 of the names in the book but get no information until they question theatrical agent Fabian Gerard, who explains that he paid back a gambling debt but was confronted by other mob enforcers claiming he didn't; when he showed them proof they left him alone.
Now the two officers figure out why Starkey was killed - he'd been pocketing debts he'd been collecting. They learn more from one of Grace Downing's recordings - Troy tells the bartender to take a package in his car's glove box and dispose of it; the bartender loaned the car to his nephew for a fishing trip and it won't be back until Wednesday, and he'll get rid of whatever this package is then.
The police, however, find and arrest Chester Davitt, and a grand jury is summoned to investigate Troy and his pals. Friday goes to get Jesse Quinn, but Quinn suddenly gets cold feet about testifying; he is supoendaed but tells the grand jury nothing substantial. The jury members question Friday in their investigation, and in the end the jury votes not to indict Troy.
Captain Hamilton angrilly orders Friday and Smith to tail Troy day and night, and they do, constantly searching him wherever he goes. It goes too far when they tail him to a gin game and several toughs attack Friday and Smith; the two policemen win the ensuing brawl but get a mild reprimand from Hamilton for it.
More pressing is a call from Grace Downing that she found something from the bartender's car at The Red Spot, but that Troy may be on to her. Friday and Smith arrive at the bar and escort her safely out, then she takes them to the back alley where they find a box of shotgun shells - with four missing.
Eventually a wiretap is installed at The Red Spot, and one of Troy's pals, Wesley Cannon, gets a call from Cleveland, and informs the bartender that Chester Davitt has been killed to keep him quiet - permanently - about the Starkey murder. Friday and Smith reluctantly confront Belle Davitt with the resulting tape, and she tearfully changes her tune, showing the officers the toolbox where Troy and Chester sawed off the shotgun barrel, and she reveals that the gun was thrown into Echo Park Lake.
With a signed affidavit from Belle Davitt and the resulting evidence, DA Alexander has enough evidence to convict Troy, but Troy pulls a final fast one that ensures he'll never be arrested.