Full spoilers follow.
Wade Vogel finds his teenage daughter Maggie in a quarantined city after she has run away — she left precisely to spare her family from watching her die, having already been bitten. A doctor who is a friend of the family allows Wade to take her home under an explicit condition: when the disease reaches its terminal phase, she must be delivered to quarantine. Everyone involved understands what quarantine means, and the film makes clear it is not treatment.
The middle movement is a decline, not a plot. Maggie's veins blacken, her wound rots, her senses dull, her appetite changes. Caroline sends the younger children away to relatives. Maggie sees friends — including other infected teenagers living out their last days — and the ordinary business of adolescence proceeds in parallel with the disease, which is the source of most of the film's pain. Neighbors and law enforcement apply steady pressure on Wade to do what rural people do with dying animals, and the film shows him confronting exactly what that would require of him.
The three endings the film refuses. Wade will not surrender her to quarantine. He cannot bring himself to kill her, though he is armed and the option is constantly present. And Maggie, seeing both the deputies circling and the ruin it would make of her father, will not let either happen.
The ending. With the final stage of the infection arriving, Maggie leaves her sleeping father, climbs onto the roof of the farmhouse, and lets herself fall. She dies on her own terms and by her own hand. In her last moments she sees herself as a small child, running through flowers with her mother — the film's single image of warmth, sunlight and innocence, in a film that has otherwise withheld the sun entirely.
What becomes clear in retrospect. Wade's sleep is the hinge of the whole film. Having spent the entire running time being unable to act, his final inaction is the only mercy available to him — whether he consciously permits it or is simply broken by exhaustion is deliberately unreadable, and the ambiguity is the point. Maggie's leap also retroactively reframes her flight to the city in the opening: both are the same act, a daughter removing herself to protect her father from having to choose. And the vision of the mother reveals what the infection has really been doing all along — not creating a monster, but taking back a child. The title is not a warning label; it is an insistence that she remained Maggie to the last second.