FDNY veterans Chuck Levine, a womanizing bachelor, and Larry Valentine, a widower father, are the pride of their Brooklyn firehouse, Engine 506/Ladder 223. During a sweep of a burned building, a segment of floor collapses on Chuck. After Larry rescues him, Chuck vows to repay him. Experiencing an epiphany from the incident, Larry tries to increase his life insurance policy but discovers that a paperwork lapse after his wife Paula's death one year earlier prevents him from naming his children, Eric and Tori, as primary beneficiaries. The insurance company representative suggests that Larry find a new spouse that he can name as his beneficiary.
Inspired by a newspaper article about domestic partnerships, Larry asks Chuck to enter a civil union with him. Chuck is reminded of his debt to Larry and agrees, entering a domestic partnership and becoming Larry's primary beneficiary. However, investigators inquire about their abrupt partnership, suspecting fraud, so the duo enlists the help of lawyer Alex McDonough, who suggests they have a formal wedding ceremony to prove their commitment. The pair travel to Niagara Falls, Ontario, Canada for a quick same-sex marriage at a wedding chapel, and Chuck moves in with Larry and his children.
Alex invites the couple to a gay benefit costume party. At the end of the evening, homophobic protesters confront the partygoers. Chuck is provoked into punching their leader, and the local news picks up the incident. With their apparent homosexuality and marriage revealed, Chuck and Larry are heckled, their fellow FDNY firefighters abandon them, and their captain, Phineas J. Tucker, aware of the truth, refuses to protect them. Their only ally is Fred G. Duncan, an angry firefighter who surprises Chuck by admitting he has not felt comfortable disclosing his homosexuality.
Chuck becomes romantically interested in Alex after the two spend time together but finds himself unable to get close to her because she thinks he is gay. During an intimate talk about relationships, the two spontaneously kiss, but Alex, still believing Chuck is gay and married, is shocked and distances herself from Chuck. Meanwhile, city agent Clinton Fitzer investigates the couple, and the strain on Larry and Chuck results in a verbal fight. Larry asserts that Chuck's constant absence to spend time with Alex is jeopardizing their ability to maintain the ruse of their relationship, and Chuck responds by instructing Larry to move on from Paula's death. Later that evening, a petition circulates to have Chuck and Larry thrown out of the firehouse. Larry confronts the crew about personal embarrassments on the job that Chuck and Larry helped them overcome. Afterward, Chuck and Larry reconcile with each other.
Eventually, numerous women publicly testify to having recently slept with Chuck, and the couple is called into court to defend their marriage against charges of fraud, with Alex as their attorney. Their fellow firefighters arrive in support, having realized all that the pair have done for them. Fitzer interrogates both men and demands they kiss to prove that their relationship is physical. Before they do so, Captain Tucker exposes their marriage as a sham and both men as straight. He then offers to be arrested as well, since he knew about the false relationship but failed to report it, which prompts the other firefighters to show solidarity by claiming a role in the wedding. The firefighters are released from jail after negotiating a deal to provide photos for an AIDS research benefit calendar, provided that Chuck and Larry plead guilty to fraud, which would reduce the charges to a misdemeanor and allow them to keep their benefits.
Two months later, Fred and Alex's brother, Kevin, marries in Niagara Falls at the same chapel as Chuck and Larry. At the wedding party (which features musical guest Lance Bass), Larry moves on from Paula's death and talks to a new woman, while Alex agrees to a dance with Chuck.
Producer Tom Shadyac had planned this film as early as 1999. I Now Pronounce You Joe and Benny, as the film was then titled, was announced as starring Nicolas Cage and Will Smith with Shadyac directing.
In the official trailer, the song "Grace Kelly" by British pop star Mika was included.