In the heart of suburban Boston, a young woman played by Julianne Hough makes a hasty exit from her home, leaving behind a bloody knife. Shortly after, she purchases a bus ticket and transforms her appearance with cropped, bleached hair. Her journey takes her to a quaint little town called Southport, North Carolina, where she adopts the name Katie. At a local general store, she embarks on a new life, securing a waitressing position and renting a modest house on the outskirts of town.
As Katie settles into her new surroundings, she forms a bond with her neighbor Jo and catches the attention of Alex Wheatley (Josh Duhamel), the manager of a nearby convenience store. Alex is a widower, still mourning the loss of his wife to cancer a few years back, and he quickly becomes smitten with Katie, performing small acts of kindness such as leaving a vintage bicycle at her doorstep. Their relationship blossoms as they take their children, Lexie and Josh, on a “family” outing to the beach, where Alex invites Katie to go canoeing—a moment that sparks their romantic connection.
While Lexie warmly embraces Katie, her brother Josh initially harbors resentment, viewing her as a threat to his mother's memory. However, he ultimately warms to her. Unbeknownst to them, a Boston police officer named Kevin Tierney (David Lyons) is on the hunt for Katie, declaring her a wanted murderer and using his position to spread her photo. Confronting Katie upon seeing her picture in the police station, Alex's trust is shattered when he discovers her true identity—Erin. The narrative unveils Erin's tragic past as an abused wife fleeing her alcoholic husband.
Soon after, Katie is seen packing her belongings during a heated discussion with Jo. When Alex arrives to speak with her, he finds her gone. In a moment of emotional clarity at the ferry, he confesses his love for her and vows to protect her. Initially torn, Katie chooses to remain in Southport, assisting Alex in the preparations for the upcoming Fourth of July festivities.
In a revealing conversation, Katie discloses to Alex that she stabbed Kevin in self-defense during one of his violent rages. Meanwhile, Kevin, in a drunken state, locates Katie's workplace and arrives just in time for the celebration, searching for her in a frenzy. Observing Katie and Alex share a dance, his anger flares as he sees Alex lean in for a kiss. That night, Katie is haunted by dreams of Jo, warning her that "he" is here.
Awakening in the convenience store next to a sleeping Lexie, Katie encounters Kevin, who is in an emotional turmoil, pleading for her to return to Boston. Despite her refusal, he becomes increasingly aggressive, brandishing a revolver and trapping her in the store, dousing it with gasoline while threatening to ignite it. In a tense moment, as Kevin contemplates his next move, Katie feigns compliance to lure him away, only to push him into the water unexpectedly.
However, a stray firework ignites the gasoline, engulfing the store in flames. As chaos ensues, Alex, busy launching fireworks, spots the fire and rushes across the lake to save his daughter. After a moment of hesitation, Lexie jumps to him. Meanwhile, a desperate struggle unfolds between Katie and Kevin, culminating in a tragic culmination as Katie grapples for the gun, resulting in Kevin's demise.
In the aftermath of the harrowing events, Alex discovers a stash of letters written by his late wife, meant for significant milestones in their children’s lives. Among them is a heartfelt letter addressed to Katie, filled with love and gratitude for rekindling Alex's spirit. The film concludes on a poignant note, revealing that Jo, who seemed to have departed, is actually the ghost of Alex's late wife, watching over them both.
