Victor Scott (Edward G. Robinson) is a district attorney with a spectacular courtoom style and a drive to win every case. He is assisted by attorney Ellen Miles (Nina Foch), who is not quite as relentless, but is devoted to her D.A. boss. They have had a long relationship: in the past, Scott was encouraged and mentored by Ellen's father, and made a deathbed promise to protect her. It is hinted that Ellen would have welcomed a romantic relationship, but instead Scott encourages her to marry a co-worker, Ray Borden (Hugh Marlowe).
After Scott discovers that a man he sent to his death is innocent, he resigns and falls into an alcoholic haze. Arrested for drunk and disorderly conduct, he determines to defend another prisoner he meets. He challenges a prosecution witness, who he says was knocked out during a brawl; when the man says a man of ordinary size could not knock him out, Scott sucker-punches him while holding a roll of coins that act like brass knuckles. The man falls unconscious, the case is dismissed, and Scott has a new career as a defense attorney.
Scott ends up defending an associate of the city's crime boss, Frank Garland (Albert Dekker), a man he refused to work for earlier because "no one would testify against you; you own the people who work for you." The man is accused of murder by poison; in the courtroom, Scott wins by drinking from the poison bottle and resting his case, knowing that the prosecution will request a recess and he can then hurry to a doctor before the poison takes effect. Though not in Garland's pocket, he establishes a careful relationship with the gangster, leading him into direct confrontation with the very office he used to head.
There is an ongoing leak between the D.A.'s office and the crime boss. The leak turns out to be Ellen's husband, Borden. Ellen discovers this, leading to a confrontation in which she kills him in self-defense. But the new D.A. (Edward Platt) gets it backward, believing that Ellen herself was the leak and that she murdered Borden when found out. She is prosecuted for murder and Scott defends her. During a lunch recess, as protection, he has his secretary take his confidential case notes and mail them to herself: if Garland kills him, they can be used to convict Garland. He then meets Garland, who asks Scott to protect him from being implicated by throwing the case. Scott assures him that he can win and also protect Garland, but is not believed. Garland has Scott followed and shot; but the hit man is himself killed by the D.A.'s agents before he can finish the job.
Rather than seeking medical treatment, Scott returns to court and calls Angel O'Hara (Jayne Mansfield), who had recently been living with Garland. Her testimony confirms that Borden spoke to Garland regularly, and made the phone call that led to Ellen learning that he was the leak. Ellen is cleared, but Scott collapses from his injuries as the movie ends.Edward G. Robinson as Victor Scott
Nina Foch as Ellen Miles
Hugh Marlowe as Ray Borden
Jayne Mansfield as Angel O'Hara (singing voice was dubbed by Bonnie Lou Williams)
Albert Dekker as Frank Garland
Howard St. John as E.A. Smith
Ellen Corby as Miss Hinkel
Edward Platt as Ralph Ford
Jan Merlin as Andy Garth
Robert Ellenstein as Joe Knight
Jay Adler as Joseph Carter
Henry Kulky as Taylor
James McCallion as Allen Parker
Addison Richards as Steve Harper
Lawrence Dobkin as Al Carol
DeForest Kelley as Edward Clary, the wrongly convicted man
During filming Robinson lent his considerable contemporary art collection to the production company. These include some impressionist works by Gauguin, Degas, Duran, and Gladys Lloyd, all of which appear in the film. Because Robinson was also the target of investigation by the House Un-American Activities Committee this movie was of a lower budget and caliber than his earlier films.
The film offered a rare serious performance by the future sex symbol, Jayne Mansfield, who went on to star in hits like: The Girl Can't Help It (1956) and Will Success Spoil Rock Hunter? (1957).
Film critic Bosley Crowther compared the film to The Asphalt Jungle but thought it was not as good. He wrote, "For one thing, the story of Illegal invades the higher echelons of crime, with a fast-thinking, double-dealing lawyer as the principal character ... The fact that this hard-bitten lawyer is played by Edward G. Robinson in his old vein of stinging sarcasm is a clue to what you may expect. But more than this and more than the climate of sordid deceit that is achieved is the fact that Illegal tries to blueprint The Asphalt Jungle's Marilyn Monroe. You may remember that Miss Monroe's first screen role was in the latter. She spoke not a word but she went right to work as an adornment in the apartment of the criminal counselor. Well, in Illegal Jayne Mansfield plays precisely the same sort of role in the apartment of Albert Dekker, the big poobah of crime. Miss Mansfield, we might add, is the beauty who is imitating Miss Monroe in a feeble imitation of Once In a Lifetime on the Broadway stage."