Jen Kornfeldt, played by Katherine Heigl, takes a trip to Nice, France, with her parents, Mr. Kornfeldt (Tom Selleck) and Mrs. Kornfeldt (Catherine O'Hara), after a breakup has left her reeling. While Mr. Kornfeldt appreciates her cautious nature, Mrs. Kornfeldt believes Jen's carefulness has left her lacking in spontaneity.
During her hotel stay, Jen encounters Spencer Aimes (played by Ashton Kutcher) in the elevator. Intrigued by him, she agrees to meet for drinks later. However, unknown to Jen, Spencer is engaged in dangerous espionage—planting a bomb on a helicopter belonging to an arms dealer. After a night of drinking, he unwittingly reveals to Jen that he is a professional assassin, shortly after she shares that she is just a regular girl seeking adventure with her family on vacation. Despite the dire warnings from his boss, Holbrook (Martin Mull), Spencer chooses love over his profession and starts a serious relationship with Jen. He surprises her father by asking for his blessing to propose, though Mr. Kornfeldt is hesitant due to Spencer’s lack of a steady job. Ultimately, Mr. Kornfeldt relents when Spencer asserts that Jen is not a fragile being and that their love does not rely upon his approval.
Fast forward three years, and we find Spencer and Jen happily married, with Spencer having started his own construction business. When Jen plans a birthday getaway to Nice for Spencer, he does not share her enthusiasm, fueling doubts among Jen's friends about his feelings for her. Things take a turn when Spencer receives a "romantic" postcard from Holbrook that threatens a return to assassin work, leaving him torn. Jen's suspicions grow when she notices Spencer’s distraction during a surprise birthday party, culminating in a hasty goodbye on her business trip.
Deciding against leaving, Jen returns home just in time to discover Spencer in a struggle with his best friend, Henry (Rob Riggle), who is also secretly an assassin. Henry reveals that a hit worth $20 million has been placed on Spencer’s head, and everyone in his life could be a potential killer. A sudden sniper attack forces Spencer and Jen to flee, leading to an explosive confrontation that ends with Henry’s demise.
As they piece together the chaos following Holbrook's death, Jen starts to suspect she might be pregnant, further complicating their dangerous lives. Despite her concerns, Spencer wishes to report the assassination, only to realize he’s been cut off from facing the truth of Holbrook’s murder. In a series of intense confrontations, they encounter various threats, including a manipulative secretary, and ultimately reveal the ties that bind their fates.
The climactic scene ensues when Mr. Kornfeldt arrives, bringing shocking revelations of his own. He had orchestrated the chaos, embedding assassins into their lives to protect Jen. Tensions rise as Spencer drops his weapon, affirming his choice to leave behind his past. Jen discloses her pregnancy to her parents, leading to a moment of reconciliation, as they all strive to build trust anew.
In the end, months later, as Spencer adopts a mustache reminiscent of Mr. Kornfeldt, the couple prepares to leave their child with the grandparents, now living a life filled with love and, humorously, booby traps to ensure their little one’s safety amidst their unconventional lives.
