In the flashback scenes that follow, Paxton plays the father of the two boys. A single father whose wife died in childbirth, he looks after his boys as best he can and seems to have forged a stable, happy family life for the three of them. Early scenes show him to be an attentive, caring father who loves his sons, and they dote on him in return. This is an especially important detail for the audience to ponder in order for what happens later to have any real resonance.
One night, the father bursts into the boys' bedroom and explains that he has been visited by an angel who has tasked them with hunting down and destroying demons in human form. The boys are baffled at first, but then one of them becomes skeptical towards his father's claims while the other supports them wholeheartedly. The flashbacks then center around what the skeptical son does in response to what he believes is his father's growing madness.
I hesitate to say anything further since part of the film functions as a thriller with plot twists and surprise reveals. But, I do want to point out that Frailty is a horror film in the truest sense of the word and one that is masterfully directed by Paxton. Rather than relying on graphic violence, Paxton achieves a pervasive mood throughout the narrative that is a mixture of fear and sadness. And, all of it is centered around a once happy family starting to crumble apart.
The final moments of the film leave it open for debate about why certain events occurred. However, it is a debate that won't offer up an easy side to be on because it largely depends on how much of an optimistic outlook one has about life and how much faith one has that rational thought is a guiding principle in the world. If you decide to believe in a more rational explanation, you have to accept some pretty unlikely coincidences; if you lean towards a supernatural cause, you have to believe that the God as presented by Judeo-Christian beliefs does not exist - at least not within the world of this film.