The Warning I Haven't Sent Yet

Ethan Blackwell wakes up to a text message sent from his own phone - timestamped twenty-four hours in the future. The message contains six words: "Don't let Mara in." He doesn't know anyone named Mara. Not yet. Over the next day, as the hours tick toward the moment his future self warned him about, Ethan starts noticing a woman who seems to know him, a best friend named Daniel Cross who insists she's harmless, and a growing certainty that the message wasn't a warning about a person at all - it was a warning about what happens after she's already inside.
As the clock runs out, Ethan has to decide whether to trust a message from a version of himself he hasn't become yet, or the woman standing at his door right now.