“I just don’t know why you want to do this” is not the reaction you’re hoping to hear when you tell your mom a major life decision, but it’s the one Megan (Kate Mara, “House of Cards”) gets after announcing that she’s joining the Marines in a newly released trailer for Gabriela Cowperthwaite’s “Megan Leavey.” “You don’t really connect with people very well,” her mom (Edie Falco, “Nurse Jackie”) observes. And that may be true — but Megan seems to have no problem connecting to another kind of ally when she’s assigned to the K9 unit.

Based on a true story, “Megan Leavey” follows “a young marine corporal whose unique discipline and bond with her military combat dog saved many lives during their deployment in Iraq,” the film’s official synopsis details. “When she is assigned to clean up the K9 unit after a disciplinary hearing, Leavey identifies with a particularly aggressive dog, Rex, and is given the chance to train him. Over the course of their service, Megan and Rex completed more than 100 missions until an IED explosion injures them, putting their fate in jeopardy.”

The intense trailer emphasizes Megan and Rex’s special bond and the marine’s mission to adopt him. When asked what she’d like to tell Rex, Megan says, “I’d thank him for teaching me what love is.”

“Megan Leavey” marks Cowperthwaite’s narrative debut. She most recently helmed 2013’s “Blackfish,” a critically acclaimed documentary about how captivity affects killer whales.

Mara’s credits include “The Martian,” “Fantastic Four,” and “American Horror Story.” “Mercy,” currently in post-production, sees her starring alongside Ellen Page. The upcoming romantic drama is directed by Tali Shalom-Ezer (“Princess”).

“Megan Leavey” opens June 9. The film is written by Annie Mumulo (“Joy,” “Bridesmaids”), Pamela Gray (“Borderline Talent”), and newcomer Tim Lovestedt.