Peter, Michael, and Jack are living happily together with Mary, who is now five, and her mother, Sylvia. Peter and Michael continue as an architect and cartoonist, while Jack has little acting work. Sylvia has become a famous actress and is dating director Edward who wishes to marry her, but Sylvia is unsure how it will affect Mary.
Sylvia and Peter are clearly in love with each other, although he won't admit his true feelings. When visiting, Sylvia's mother warns her that he may never be able to express or admit his feelings. Sylvia, realizing she wants to get married and start a family, accepts Edward's proposal, announcing she and Mary will be moving to the UK after the wedding.
Inviting Edward to the apartment, Peter tells him he believes he won't be a good father for Mary. When he leaves, Sylvia confronts Peter, leading to a falling out when she calls him selfish, and he reminds her she abandoned Mary once (as described in the first film).
Sylvia and Mary leave the next day for the UK. The men, depressed, try to cheer themselves up with one of their bachelor-style parties, but are still miserable without Mary and Sylvia. They go to the UK to visit Mary, who is unhappy without them. Peter and Michael arrive in time for the rehearsal dinner, happily reuniting with Mary and Sylvia. Miss Elspeth Lomax, headmistress of Pileforth Academy for Girls, is introduced to Peter by Edward (who tells her Peter is secretly interested in her). Peter and Sylvia apologize to each other for the fight.
With the wedding imminent, Peter is concerned as Mary says Edward dislikes her. Peter and Michael realize Edward plans to send Mary to Pileforth boarding school. Edward denies it and Sylvia refuses to believe Peter, as he has always disliked him. Jack arrives mid-argument and Sylvia and Edward leave. Peter admits he loves Sylvia but stayed silent because of him. Jack insists that Sylvia only loves Peter and must follow his heart.
The night before the wedding, Peter goes to Pileforth to get proof of Edward's scheme. Elspeth believes Peter is admitting his "feelings", throwing herself at him. Very surprised and deflecting her advances, he gets away. His car breaks down, finally he calls Jack and Michael, confirming he has the proof, but he will be late. Michael, Jack and Mary try to stall the wedding. Michael kidnaps the vicar and Jack disguises himself as an elderly replacement. Peter, with help from Elspeth, heads to the wedding. During the ride, she says Edward told her Peter was interested in her, but Peter says Edward lied, apologizing for him.
After numerous delays they arrive at the church. Peter shows Sylvia the truth, Elspeth confirming that Edward has been lying and he, trying to defend himself, prompts Mary to accuse him of lying again. He shows his true colors, swearing at Mary and Peter, who then punches him out. Sylvia insists she's going home, but Peter stops her, ultimately declaring his love. Then, Edward regains consciousness, stating it is too late as they are already married. Jack then reveals himself – he has both finally proven his acting skills, and the marriage is not valid.
Peter and Sylvia wed with Mary as their bridesmaid, who afterwards throws the bouquet into the air; it is caught by a shocked Jack.
Tom Selleck as Peter Mitchell, One of the 3 men, a successful NYC architect
Steve Guttenberg as Michael Kellam, One of the 3 men, a cartoonist
Ted Danson as Jack Holden, One of the 3 men, an actor and Mary's birth father
Nancy Travis as Sylvia Bennington-Mitchell, Mary's mother
Christopher Cazenove as Edward Hargreave, a successful director and the scheming bachelor
Fiona Shaw as Miss Elspeth Lomax, headmistress of Pileforth Academy for Girls
Robin Weisman as Mary Bennington, Sylvia's daughter
John Boswall as Barrow, Edward's butler
Sheila Hancock as Vera Bennington, Sylvia's mother
the sequel to the 1987 film Three Men and a Baby