U.S. Navy SEAL Lieutenant Shane Wolfe (Vin Diesel) is put on a mission to extract Howard Plummer (Tate Donovan), a man involved in a highly classified government project, from a group of Serbian rebels. Though Wolfe's team successfully retrieves Plummer from an enemy vessel, chaos ensues as both men come under fire while making their escape. Tragically, Plummer is killed, and Wolfe is left to recuperate in the hospital for two months.
Once he recovers, Wolfe's commanding officer, Captain Bill Fawcett (Chris Potter), tasks him with looking after Plummer's widow, Julie (Faith Ford), and their five children at home in Bethesda, Maryland. The mission builds around searching for the secret project known as GHOST, hidden within the family's residence. The children—Zoe (Brittany Snow), Seth (Max Thieriot), Lulu (Morgan York), Peter (Logan and Keegan Hoover), and Baby Tyler (Bo and Luke Vink)—prove to be a handful, making Wolfe's task all the more challenging, especially with the additional assistance of nanny Helga (Carol Kane).
Wolfe quickly realizes that the children are not the only ones who rebel against his efforts; Zoe and Seth continuously push back, Lulu forms an attachment to him, and even the family pet duck, Gary, shows its disdain. The situation escalates when a prank goes awry, leading Helga to resign. Amidst all this chaos, Wolfe discovers Lulu and her friends, the Fireflies (Jordan Todosey, Nikki Shah, Maria Georgas, and Emi Yaguchi-Chow), trying to raise money with cookies at a Costco, where they’re interrupted by rowdy rival boy scouts, resulting in a hilarious disaster.
Upon returning home, Wolfe finds a surprise house party organized by Zoe and her boyfriend, Scott (Kyle Schmid). Determined to restore order, he enforces a cleaning session for the guests. Feeling unappreciated for the children’s grief over their father, Zoe confronts Wolfe, leading to unexpected mayhem when they’re suddenly attacked by a pair of masked ninjas, which Wolfe easily overcomes. As he clarifies the scenario to the kids, they agree to cooperate, creating an unexpected partnership.
Not long after, Vice Principal Duane Murney (Brad Garrett) brings troubling news of Seth's erratic behavior, prompting Wolfe to investigate further. Seth, in a rebellious outburst, confesses he only joined the wrestling team to meet his father's expectations and sneaks off to participate in an amateur theater production of "The Sound of Music." When the original director quits, Wolfe steps in to help, balancing the production with managing the household and imparting self-defense skills to the Fireflies.
Eventually, encouraged by Wolfe, Seth leaves the wrestling team to embrace his passion for theater. Just when everything seems to align, Murney corners Seth, leading to a humorous wrestling confrontation where Wolfe emerges victorious. The Firefly Scouts’ training pays off when they confront their bully rivals yet again, turning the tables on them.
Amidst shared stories about their fathers’ pasts, a phone call from Julie reveals she has deciphered a hidden code leading to a significant key. The children swiftly prepare for her return; however, a clandestine vault and a shock reveal of betrayal from Fawcett complicates the mission when he knocks out Wolfe and restrains the children.
In a triumph of teamwork, the kids manage to free themselves and summon help while Wolfe attempts to aid Julie in accessing the vault, utilizing a quirky dance to bypass the security system. An epic clash with the Chuns—a pair of double-crossers—ensues, leading to a showdown where Wolfe ultimately prevails, with Julie delivering the final blow against Mrs. Chun.
As the police swarm the house and capture the culprits, a delightful twist reveals a romantic connection budding between Wolfe and Claire Fletcher (Lauren Graham), leading to a stolen kiss. With the danger past, Wolfe seamlessly integrates into the family, taking on the role of the new wrestling coach as the film wraps up at Seth's triumphant stage debut, showcasing a heartfelt transition from soldier to family protector. The last scene finds Murney humorously belting out "Climb Every Mountain," clad in a nun's costume, bringing the adventure to a close.
