Lt. Frank Ballinger is a no-nonsense plain clothes cop in the elite M Squad Division. The Squad's task is to root out organised crime and corruption in America's Second City, Chicago.
The Squad is on the lookout for a bank robbery suspect with a unique feature, witnesses say he has gold teeth.
Ballinger and the Squad are out to catch a burglar who may have rabies.
The squad hunts down a cop killer.
The squad goes after a jail inmate who's just escaped.
Ballinger investigates the murders of four women. The victims were all connected to the same church and lived within two miles of each other.
Chicago cop Frank Ballinger stages a rescue in order to gain the confidence of Alice, a young woman who works for suspected jewelry fence Louis Prince. Pretending to be furniture salesman Bob Steele, Ballinger romances Alice and gets friendly with her boss. But Alice falls for "Bob" and Ballinger must weigh her feelings and safety against his duty as a police officer.
Lt. Frank Ballinger investigates the case of a gambling ring and its connection to an unidentified police officer. The deeper into the investigation he gets, the more Ballinger becomes concerned that the crooked cop may be an old friend.
A jewelry store robbery sends the thief to prison, but his claim that the night watchman, a former big-league ballplayer named Lightnin' Joe Adams, was in on the heist costs Adams his job. Lt. Frank Ballinger goes undercover in prison to find out from the robber whether Adams was involved in the crime and what happened to the loot.
Frank Ballinger and his colleagues on the M Squad have to find a needle in a haystack: a cop killer somewhere in the city of Chicago. Their only clues are an old magazine and the killer's habit of saying "You bet."
Lt. Frank Ballinger keeps an eye on Hazel, a bar girl with a record who nonetheless wants to go straight. He discovers she is involved with Joe Mazzerin, a safecracker. Ballinger goes undercover to get the goods on Mazzerin and to learn who his mysterious partner is.
When perennial loser Wally Gardner is identified as the perpetrator of a robbery-murder, he tells police unique details of a stranger he met miles away at the time of the crime. But when that stranger is found, he says he can prove that HE was miles from where Gardner says they met at the time of the crime. Lt. Frank Ballinger must determine who's lying.
When a hired gunman gets himself tossed in jail following a fistfight, Lt. Frank Ballinger takes his place undercover as protection for a group of three businessmen who meet once a year to commit a specialized crime.
When his partner in an investigation is killed, Lt. Frank Ballinger makes it a personal business to pin the crime on narcotics lord Sam Hinder. But, as Ballinger discovers, Hinder has an unusual reason for wanting to keep the police out of his own personal business.
Gangster Tommy Hatch is wanted for murder, but the police have to keep him safe from other gangsters who want him dead before he can testify against them. Lt. Frank Ballinger is assigned to protect Hatch even though he despises him.
Ballinger investigates the case of a security courier who is charged with robbing himself of $75,000.
District Attorney Grover calls for Ballinger to help him sort out a potentially embarrassing situation involving the murder of a former business associate.
The room mate of the third murder victim of the Blue Indigo serial killer may be the only link Ballinger has to catching him.
When a convicted murderer gets the upper hand on Lieutenant Ballinger during a train ride back to Chicago, his last best hope for recapturing the felon is the persistence of an alert young boy.
Ballinger suspects an inside job when a dry cleaning company refuses to knuckle under to a shakedown mob.
Kathy Bane, an old flame of Ballinger's, needs his help when a blackmailer trying to extort money turns up dead-- in her apartment.
Ballinger investigates a murder with multiple motives.
A man's aunt reports her nephew is missing. Ballinger investigates.
Ballinger goes undercover to capture a robbery suspect.
Ballinger investigates after a bank robber leaves a pencil at the scene.
A woman dies. Ballinger investigates.
A woman loses her memory. Bellinger investigates.
Bank robbers hide in a suburban house.
A sniper is terrorizing Chicago. Ballinger joins the hunt.
A young wife is shot by an unknown assailant when she drives with her ex-con husband to an isolated woods in search of their dream home. No one even reports the couple missing until after the police find them dead in the snow. The wife's sister is a wealthy spinster, who takes her time even coming to Chicago, claiming her brother-in-law was threatened with death by a blackmailer. Det. Ballinger suspects the sister knows more than she reveals.
A boxer's (Lancaster) life is being threatened soon after another fighter dies after fighting with Lancaster. Suspicion falls on the other fighter's trainer who feels Lancaster is responsible for the death of the other fighter.
A man confesses to a crime he may not have committed. Meanwhile the president of the company where the man works has been shot to death.
Lieutenant Ballinger searches for a mentally ill young woman who shot a psychiatrist, hoping all the while that he can also find sufficient evidence to obtain a felony conviction against the charlatan who was "treating" her.
Frank Ballinger investigates a illegal adoption racket.
Ballinger investigates a rash of counterfeit $20 bills.
A professional man is identified as a jewelry-store robber, threatening his freedom, not to mention his wife's reputation "from being married unknowingly to such a character". Ballinger investigates.
An illegal roving casino eludes the police and cheats players with crooked games. When Ballinger investigates, he encounters additional, unforeseen threats.
No overview available for this episode.
A millionaire is found dead in a locked room. His young pretty wife (Fay Spain), and the big hansom driver (Ken Clark), are suspected of the crime. Lee Marvin figures out how the crime was committed.
