In Dublin in 1922, Gypo Nolan (Victor McLaglen) has been kicked out of the outlaw Irish Republican Army (IRA) for not executing a Black and Tan who killed an IRA man. He becomes angry when he sees his streetwalker girlfriend Katie Madden (Margot Grahame) trying to pick up a customer. After he throws the man into the street, Katie laments that she does not have £10 for passage to America to start afresh.
Gypo later runs into his friend and IRA comrade Frankie McPhillip (Wallace Ford), a fugitive with a £20 bounty on his head. Frankie, tired of hiding for six months, is on his way home to visit his mother (Una O'Connor) and sister Mary (Heather Angel) under cover of the foggy night. The slow-witted Gypo decides to turn informer for the £20 reward, enough for passage to America for the both of them. The Black and Tans find Frankie at his house, and Frankie is killed in the ensuing gunfight. The British contemptuously give Gypo his blood money and let him go.
Gypo subsequently buys a bottle of whiskey and tells Katie that he obtained money by beating up an American sailor. He goes to Frankie's wake, and acts suspiciously when coins fall out of his pocket. The men there tell him that they do not suspect Gypo of informing, but he then meets with several of his former IRA comrades, who wonder who informed on Frankie. Gypo claims it was a man named Mulligan (Donald Meek). Though Gypo is drunk and talking nonsense, the others begin to suspect him but do not have enough evidence as yet. Gypo leaves and gives out £1 notes to a blind man (D'Arcy Corrigan) and some bar patrons, but people wonder why he had such a sudden influx of cash. Meanwhile, Mary tells the IRA that the only person Frankie talked to that day was Gypo, and the men decide to hold an inquest into the death.
Gypo goes to an upper-class party to look for Katie, but gets drunk and buys rounds of drinks. Gypo is then taken away by his former IRA comrades when they figure out it was he. He is taken to a kangaroo court, where Mulligan is questioned and is accused once again by Gypo. However, the comrades do not believe Gypo, and give him a detailed accounting of where he spent his entire £20 reward. Gypo then confesses to ratting out Frankie.
Gypo is locked up, but before he can be executed he escapes through a hole in the ceiling. He runs to Katie's apartment, where he tells her that he informed on Frankie. Katie goes to see the commissioner who presided over the trial, Dan Gallagher (Preston Foster), to beg him to leave Gypo alone. The rigid Gallagher says he cannot do anything, and Gypo might turn in the entire organization to the police if he is allowed to live. However, other IRA members, having overheard Katie, go to her apartment and shoot Gypo, much to Katie's horror as she hears the shots. Gypo wanders into a church where Frankie's mother is praying and begs forgiveness as he confesses to her. She does forgive him, telling him that he did not know what he was doing, and the absolved Gypo dies content on the floor of the church after calling out to Frankie with joy.
Writing for The Spectator in 1935, Graham Greene praised the film as "a memorable picture" the substance of which made "superb material for the screen." Greene singled out the acting of Victor McLaglen for specific praise, noting that he had "never given an abler performance".
The film was popular at the box office, earning a profit of $325,000 and remains one of John Ford's most widely referenced films.
The film was nominated for six Academy Awards, winning four. McLaglen won Best Actor for his portrayal of Gypo Nolan, beating out Charles Laughton, Clark Gable, and Franchot Tone for the better-remembered Mutiny on the Bounty, and Ford won Best Director. Dudley Nichols won Best Writing, Adapted Screenplay, but turned it down because of union disagreements. It was the first time an Oscar was declined, though he later claimed it three years later. The film also won the Oscar for Best Score; Max Steiner won for the first time. The film was nominated for Outstanding Production, as well as for Best Film Editing.
The film's other awards and nominations:National Board of Review - Best Picture
New York Film Critics Circle Awards - Best Film and Best Director
Venice Film Festival - John Ford nominated for the Mussolini Cup
2005: AFI's 100 Years of Film Scores – Nominated
The film is recognized by American Film Institute in these lists:
The Informer was adapted as a radio play on the July 10, 1944, and October 17, 1950, episodes of The Screen Guild Theater, the March 28, 1948, episode of the Ford Theatre. On the Academy Award Theater's May 25, 1946, episode, McLaglen reprised his role.