Almost a decade has passed since a devastating pandemic decimated the majority of the female population. Amidst this backdrop, Dad (Casey Affleck) embarks on a perilous journey across British Columbia with his daughter Rag (Anna Pniowsky), who is among the very few remaining females in existence. To shield her from predatory male bandits on the hunt for women, Dad disguises Rag as his son. Haunted by the promise he made to his late wife, Mom (Elisabeth Moss), Dad vows to share with Rag all there is to know about her mother.
After a few days in a wooded campsite, they seek refuge in an abandoned house. Their brief respite is shattered when four intruders attempt to break in, forcing them to escape once more. In desperation, Dad hijacks a car to reach his grandparents' home, thinking it a safer haven.
Upon arrival, they discover that their sanctuary has been taken over by Tom (Tom Bower) and his companions, Calvin (Hrothgar Mathews) and Lemmy (Timothy Webber). Tom, a devout man, welcomes them with open arms. As evening falls, Dad and Rag stealthily relocate their possessions to a nearby cabin, a precaution for a "red alert"—a term for imminent danger that Dad employs to keep Rag cautious. The following morning, Tom teaches Rag how to handle a shotgun for protection.
Growing increasingly suspicious, Tom confronts Dad, questioning whether the "son" he sees is truly a daughter. Revealing the tragic loss of nine female relatives—eight to the virus and one to "a pack of monsters"—Tom's concern deepens. Struck by the gravity of Tom's story, Dad reveals the truth about Rag, displaying a family photograph, and explaining that Rag's full name is Anna Elizabeth, affectionately shortened to Raggedy Ann.
Tragedy strikes when three men invade their temporary home, mercilessly slaying Tom and Lemmy, with Calvin betraying them to the attackers. Rag and Dad manage to hide in the attic, and he bravely lowers her out of a window before engaging the intruders. In a desperate battle, Dad succeeds in taking down two of the assailants, but faces near defeat at the hands of the last. Just when hope seems lost, Rag fires the shotgun to save her father but unintentionally injures him in the process.
They retreat to the cabin, and Rag dutifully removes the pellet from Dad's wound. As he recuperates, a poignant flashback recalls his vow to Mom, promising to protect their daughter. Once Rag tends to his injury, Dad shares his plans to take them to the safe haven Tom spoke of. Overwhelmed with emotion, he breaks down, only to find solace in Rag's comforting embrace, echoing the love that Mom used to provide.
