Cassie Salazar (Sofia Carson) is a waitress/bartender and performs with her band (including Nora (Kat Cunning)), The Loyal, at a bar. Diagnosed with Type 1 diabetes six months earlier, she struggles to afford insulin and works multiple jobs to try and make ends meet. One night, she serves a group of Marines who are soon to be deployed to Iraq. One of them, Luke Morrow (Nicholas Galitzine), flirts with Cassie, but she turns him down. Luke has his own set of difficulties: he became an addict after his mom's death, and although two years clean, he still owes $15,000 to his dealer Johnno (Anthony Ippolito). Estranged from his retired Marine dad Jacob Morrow Sr. (Linden Ashby), he asks his brother Jacob Morrow Jr. (Scott Deckert) for help, but his brother declines.Cassie proposes to Frankie (Chosen Jacobs), a childhood friend and Luke's bunk-mate, to benefit from the health insurance granted to military spouses. Frankie explains he plans to marry his sweetheart Riley (Breana Raquel). Although Luke overhears and advises against a fraudulent marriage, he realizes both their financial difficulties could be solved by getting married as Luke will make more money (He gets extra spouse allowance) and Cassie will have access to military spouse health-care. They eventually agree to marry before Luke deploys. Their plan is that after a year, they will file for divorce.Frankie, the witness at the wedding, gives Cassie the ring he plans to marry Riley with, asking her to keep it safe. After, the newlyweds go to the bar with Frankie and Luke's fellow marines. Cassie argues with one of them, Armando (Nicholas Duvernay), over a toast about hunting down Arabs, resulting in an argument with Luke, but to keep up appearances they pretend to make amends. That night, he admits he is scared of both the marriage and Iraq. Cassie comforts him and they share an intimate night together.The next morning, the Marines are deployed. Before he gets on the bus to leave, Luke gives Cassie contact information for his brother as she is now his next of kin. Cassie and Luke start sending emails and video calls to one another to keep up the ruse. She tells him that she wrote a song, "Come Back Home," for the Marines. She performs it for them, uplifting their spirits after a rough day.The song goes viral. Luke asks Cassie if he is her muse, and Cassie concedes he may be. One night while performing, she receives a call that Luke has been severely injured after an IED went off and will be sent back. Cassie tries to contact Luke's brother but accidentally contacts their father, angering Luke, as his father is a retired MP Marine officer who would turn them in. Afterwards, it is revealed Frankie was killed in action. At his funeral, she gives Riley the ring she promised to keep safe for him.Luke, now a wheelchair user, moves into Cassie's apartment after being discharged from the hospital. Luke makes a wedding ring out of his dog tag chain for Cassie and they re-decorate their house to showcase their marriage for his father, who picks him up for physical therapy. Cassie adopts Peaches, a golden retriever, as an emotional support animal to aid in Luke's recovery. Luke's rehab and their cohabitation inspire Cassie to write another song, "I Hate the Way," which she performs fabulously at Whisky a GoGo and captures the attention of record companies.Still seeking his money (payments to Johnno had stopped ever since Luke returned to the US), Johnno breaks Cassie's mother's Marisol Salazar (Loren Escandon) window and threatens Luke. That night, Cassie's sugar level drops, sending her into shock. Luke helps her recover and they share a kiss. The next day, Luke beats Johnno up, gives him money and tells him to stay away. Instead, Johnno informs Cassie's mother about Luke's past. When Cassie confronts Luke, he reveals he had stolen his father's $50,000 car to sell but crashed it instead, forcing him to borrow money from Johnno to pay him back. Cassie asks for a divorce and demands that he leave her apartment by the next day. When Luke returns home from a run, he is detained by the MPs, who were informed by Johnno of their fraudulent marriage. Luke's father calls Cassie to tell her of the charges and the impending trial.At his trial, Luke pleads guilty, taking full responsibility and says that Cassie was unaware she violated military law. Luke is sentenced to six months in the brig after which he will receive a Bad Conduct Discharge. Cassie's band is signed to a label and are opening for Florence and the Machine at the Hollywood Bowl. During the show, on the same day that Luke is to be jailed, Cassie sings her newest Luke-inspired song, "I Didn't Know," which was apparently written while waiting for Luke's trial. After the show, she rushes to confess her love for Luke before he is sent off. Luke gives her his wedding ring and tells her "it's real now." As the credits play out, six months later, Luke and Cassie are a happily married couple at the beach where they're enjoying a picnic with Peaches.