Based on actual events, Megan Leavey is set in 2003 and follows the journey of an aimless teenager, Megan (played by Kate Mara), from upstate New York. Following the tragic overdose of her best friend, Megan feels trapped in her small town, burdened by a difficult relationship with her mother, Jackie (portrayed by Edie Falco). Seeking a fresh start, she enlists in the US Marines.

After struggling through basic training, she finds herself at Camp Pendleton, where she quickly encounters trouble. As punishment, she’s tasked with cleaning the enclosures of the K-9 bomb detection dogs. It’s here that she meets an aggressive German Shepherd named Rex. Despite her fear, she approaches Gunnery Sergeant Martin (played by Common), expressing her desire to become a dog handler. He agrees, but due to a shortage of dogs, she must practice using an ammunition box instead.

As months roll on, Megan faces a bittersweet moment when she is finally assigned to Rex, who has a history of injuring his previous handler. With limited time before their deployment to Iraq, Megan and Rex must bond quickly. Initially challenged by Rex’s obstinacy, their relationship strengthens as she allows him into her living quarters rather than the kennel. During this period, she also starts dating Matt Morales (played by Ramón Rodríguez), a fellow handler.

Fast forward to 2005, and the K-9 unit is deployed in Iraq. Megan and Rex participate in several missions, aiding their fellow Marines while overcoming initial setbacks. Together, they manage to uncover a major weapons cache, significantly contributing to the safety of countless troops by detecting numerous IEDs. However, during a mission in Ramadi, disaster strikes as they are caught in an explosion caused by a hidden IED. Miraculously, both Megan and Rex survive and recuperate at their base.

Returning to Camp Pendleton, Megan makes the difficult decision to not re-enlist. Believing that Rex will remain with her since he’s injured, she expresses her wish to adopt him. Sergeant Martin supports her, yet she learns that Rex will be reassigned with a new handler despite his injuries. This news devastates Megan, compounded by her struggle with PTSD from the blast.

Facing overwhelming emotions, she ends her relationship with Matt, who chooses to re-enlist, and engages in a heated argument with her mother, which leads her to move in with her father, Bob (played by Bradley Whitford). Bob encourages her to take action instead of wallowing in despair, igniting a fire within her. Megan then launches a campaign advocating for all military handlers to retire their service dogs, gathering support through petitions, media appearances, and help from US Senator Chuck Schumer.

By 2009, Megan discovers Rex is being retired due to health issues. She attends his retirement ceremony at Camp Pendleton, where she reunites with Matt, who brings good news about her adoption request being approved by the Marines. Their emotional reunion with Rex brings closure and joy to her previous struggles.

Finally, the film culminates with a heartwarming tribute at Yankee Stadium, where the New York Yankees honor both Megan and Rex in front of a cheering crowd, including her proud parents. The story ends with clips of the real Megan Leavey and Rex, noting that Rex lived for three more years after his retirement. Today, Megan is a dedicated dog handler and trainer, continuing her important work in bomb detection at high-profile venues, including the United Nations.