The three bachelors - architect Peter Mitchell (played by Tom Selleck), actor Michael Kellam (played by Steve Guttenberg), and cartoonist Jack Holden (played by Ted Danson) - live together in a luxurious New York City apartment and enjoy their bachelor lifestyle. But their lives are turned upside down when a baby girl named Mary is left on their doorstep by Jack's girlfriend, who has to leave suddenly for a job in Europe. They initially struggle to take care of the baby, with comical mishaps such as being unable to change a diaper and accidentally buying feminine hygiene products for her.
As they slowly adapt to their new responsibilities and begin to bond with Mary, they face challenges such as handling her crying fits, pacifying her during a fever, and trying to keep their newfound parenting skills a secret from their friends and colleagues. Meanwhile, Jack's girlfriend returns unexpectedly and wants to take Mary with her to Europe. The three men must decide whether to hand over Mary to her mother or fight to keep her in their lives.
In the end, they come to realize that they have grown attached to Mary and have become a family, and they fight to keep her in their care. As they celebrate Mary's first birthday, they reflect on how much she has changed their lives and how much they have learned about themselves and their capacity for love and responsibility.