After a successful sting operation-turned-shootout, LAPD Sergeant Joe Bomowski [Sylvester Stallone] tries to call his elderly mother, Tutti Bomowski [Estelle Getty] at home in Newark, New Jersey. He is dating his superior, Lieutenant Gwen Harper [JoBeth Williams], who accuses him of calling another woman and ends their relationship. His mother arrives in Los Angeles for a visit, and Joe is mortified by the well-meaning but overbearing Tutti, who embarrasses him even while talking a suicidal man off a ledge.
Disapproving of Joe's bachelor life, Tutti tidies up his apartment and ruins his sidearm with bleach. She visits a gun store to buy a replacement only to learn there is a mandatory “cooling off” period, and another customer, Mitchell [Dennis Burkley], and his brother sell her an illegal MAC-10 machine pistol instead. Shady businessman Parnell [Roger Rees] has sent his henchmen, Paulie [Martin Ferrero] and Munroe [Gailard Sartain], to recover guns stolen by Mitchell, who drives off after they open fire and kill his brother.
As the only witness to the murder, Tutti withholds information from Joe's rival Ross [J. Kenneth Campbell], determined to help her son solve the case himself and repair his relationship with Gwen. Joe has a nightmare about being babied by his mother, who presents him with the MAC-10. Tutti commiserates with Gwen over Joe's inability to share his emotions, and gives her son a full description of Mitchell and his van. Interrogating Mitchell's girlfriend at his apartment, the henchmen flee when Joe arrives with Tutti, who gives chase but crashes Joe's car.
Joe gives Ross his mother's information, but subjects him to a swirly when he disrespects her. The MAC-10 is traced to a shipment of weapons supposedly lost in a suspicious warehouse fire, which earned Parnell a $12 million insurance payout. Questioned by Joe and Tutti, Parnell orders his men to eliminate them. As Joe and his mother investigate the derelict warehouse, the henchmen attack them with a wrecking ball, and Munroe is shot at by Tutti and arrested by Joe.
Gwen kisses Joe for sending her a roomful of roses, but slaps him when he realizes that his mother was responsible, and he angrily confronts Tutti for interfering with his life. Preparing to sell the weapons in South America, Parnell has Paulie thrown out a window to his death. After his mother's advice allows him to reconcile with Gwen, Joe finds an apology letter from Tutti, who has left for the airport. He races to thank her, and she reluctantly agrees to abide by his rules for the rest of her stay.
Mitchell breaks into Joe's apartment and Tutti knocks him out with a frying pan, but he is seeking Joe's help, revealing that the guns he stole were from the “lost” shipment Parnell has hidden at an airstrip. Leaving Tutti and Mitchell handcuffed at his apartment, Joe arrives at the airstrip as Parnell loads the weapons onto a waiting plane. Tutti slips free and knocks Mitchell out again, taking his gun and van to the airstrip, where she is taken hostage. Stopping the plane with a truck, Joe is captured and Parnell prepares to kill him, but is shot in the shoulder by Tutti as the police arrive.
At the end of Tutti's visit, Joe and Gwen, now newly engaged, see her off at the airport. She recognizes a man from America’s Most Wanted and Joe chases him down, as Tutti remembers he was on the run for shooting his own mother.
