The black-and-white film is set in New York City and stars Ray Milland and Maureen O'Sullivan. Elsa Lanchester and Charles Laughton appear in the film, as does Harry Morgan, in an early film role, listed in the credits as Henry Morgan, as a hired thug. Noel Neill has an uncredited part as an elevator operator very early in the film.
The story is told in flashback. The opening shows George Stroud (Ray Milland), editor-in-chief of Crimeways magazine, shown hiding from building security inside the "big clock" ― the largest and most sophisticated clock ever built, which dominates the lobby of the Janoth Publications building in New York City, where he works.
The flashback then starts a day earlier: Stroud is eager to go on a long-postponed vacation in West Virginia with his wife Georgette (Maureen O'Sullivan) and son. His tyrannical boss, Earl Janoth (Charles Laughton) wants him to stay, and follow up on a missing persons story Stroud has just cracked, but Stroud refuses and Janoth fires him. Instead of meeting Georgette at the train station as planned, however, Stroud is distracted by the attentions of Janoth's glamorous mistress, Pauline York (Rita Johnson), who proposes a blackmail plan against Janoth. When Stroud misses their scheduled train, Georgette angrily leaves without him. Stroud begins drinking and spends the evening out on the town with York at various locations, buying a painting and a sundial.
Stroud and York go to York's apartment, but she sees Janoth arriving, and Stroud leaves. Janoth sees him leaving but does not recognize Stroud in the dark. Although Stroud's evening with York had ended platonically, Janoth assumes otherwise, leading to a quarrel which ends with him striking York with the sundial and killing her. Janoth goes to his assistant, Hagen (George Macready) and tells him what happened, intending on going to the police and confessing. But Hagen talks him out of it and convinces him that they can frame the man Janoth saw leaving York's apartment for the crime. Janoth decides to use the resources of Crimeways to find the man instead of calling the police.
Stroud has since caught up with his wife and son in West Virginia and tells her that he has been fired (but leaves out his adventures with York). Janoth calls to re-hire him, to lead the effort to find the mystery man (leaving out any mention of York), He mentions enough details for Stroud to know that the mystery man is himself. He reluctantly agrees to return to his job and lead the manhunt, to Georgette's disappointment.
During the manhunt, Stroud has to appear to lead the investigation diligently, and at the same time, prevent the investigation identifying him as its target. Meanwhile, he must also secretly carry out his own investigation to find the real murderer.
Eventually York is identified and witnesses are found that saw her out on the town wiht the mystery man. These witnesses are brought to the Janoth Building.
One is eccentric artist Louise Patterson (Elsa Lanchester), who did the painting that was purchased by Stroud. Asked to paint a portrait of the mystery man, she produces a modernist abstract of blobs and swirls.
Stroud tries to avoid the witneesses, but one of them sees and recognizes him as the mystery man. Stroud slips away before the witness points him out to the investigators; but now the investigators know that the mystery man is in the building, though not who he is. All exits from the building are sealed, and everyone must leave by the main door, with the witnesses watching for the mystery man. Building security men sweep the building to flush out the wanted man.
Stroud evades the dragnet by various maneuvers, finally hiding in the clock (ending the flashback segment of the film).
In the climax of the film, Stround confronts Janoth and Hagen. He presents evidence which appears to point to Hagen as the killer. (Hagen was Janoth's go-between with York.) Hagen implores Janoth to clear him, but Janoth tells him only that he will provide him the best possible legal defense. Enraged, Hagen reveals that Janoth killed York and he helped cover it up. Janoth shoots Hagen (apparently killing him) and flees. Janoth tries to escape in an elevator, but the elevator car is stuck floors below (jammed there by Stroud earlier while evading the security men); Janoth falls down the elevator shaft to his death.
Morgan's screen name later would become "Henry 'Harry' Morgan" and eventually Harry Morgan, to avoid confusion with the popular humorist of the same name.
Paramount bought the rights to the novel before publication. (Fearing's earlier novel The Hospital had been a best seller.) The purchase price was a reported $45,000.
Jonathan Latimer was assigned to write the script and Ray Milland to star. Leslie Fenton was announced as director but he was held up on Saigon so John Farrow took over. Filming began February 17, 1947. Charles Laughton was cast as the villain.
This was Maureen O'Sullivan's first film in five years, since Tarzan's New York Adventure, after which she had concentrated on raising her family. She did it as a favor for her husband, director John Farrow.
Film critic Bruce Eder wrote, The Big Clock is a near-perfect match for the book, telling in generally superb visual style a tale set against the backdrop of upscale 1940s New York and offering an early (but accurate) depiction of the modern media industry."
Film writer David M. Meyer calls The Big Clock, "More screwball comedy than noir, The Big Clock's big moments derive from snappy dialogue and over-the-top humor."
In 2001, the American Film Institute nominated this film for AFI's 100 Years...100 Thrills.
The story was remade in 1987 as No Way Out with Kevin Costner. The 1948 film is closer to the novel. The 1987 remake, on the other hand, updated the events to the United States Department of Defense in Washington D.C. during the Cold War.