Katherine Winter (Hilary Swank) is a professor at Louisiana State University. She teaches about how she disproves religious miracles. It turns out, by many flash backs through out the movie, that she was once an ordained minister who had a husband and a young daughter. She lost all faith in God when the village she was staying at in Africa decided to kill her family because they were "the cause" of the current drought there.
A town called Haven had a river that turned red (P1). The town's people thought it was caused by the local "crazy" family, the McConnells. They found young Loren McConnell (Anna Sophia Robb) down by the river with her dead older brother. Doug (David Morrissey) came to Katherine with what happened and asked for her to help solve their problem. Her ex-Teacher Assistant/ best friend Ben (Idris Elba) and herself go to Haven. After wading through the river, frogs falling from the trees (P2), and collecting samples from the river they conclude that the river really is human blood. Kate and Ben stay at Doug's house while in Haven. While there all the children get their hair shaved off because the lice becomes so bad (P3). One night while Doug is BBQing flies and maggots swarm their food (P4). Ben collects samples of the flies. A local farmer's livestock all start to die (P5). Kate collects samples of the livestock. Ben finds, through some computer analysis, that the livestock cells were healthy and they should not be dying. Kate goes out to the McConnell's farm and realize that Loren is no ordinary girl. Her mother asks Kate if she is going to kill Loren, Kate says no, mother asks why not, etc. (just like commercial). Apparently, there is this cult that puts a symbol on the body of their first born child and I guess sacrifice their second child because they are evil? At this point it's obvious that Loren is controlling the plagues. She gets pissed that the towns people are trying to kill her so she brings locusts who surrounds and kill the people (P8). Also, people start dying of boils (P6). Plagues 7 and 9 do not come until the end when Kate touches Loren with the intent on killing her because she thinks she is evil. She gets a flashback that we see Loren's older brother, along with everyone else from the town, was going to sacrifice her by stabbing her in the heart. When he stabbed her, she didnt die, he did.
As it turns out, Loren is an Angel from God who is causing the plagues on the town because of what they are doing. And it turns out Doug is in on the sacrificing too. He killed Ben. In the end, because the towns people were unable to stop Loren, she brings the 10th Plague and everyone in the town dies, including Doug, because they are all first born (due to the second children sacrificing from earlier. apparently this has been happening for many generations).
As Katherine drives off, she tells Loren that it's just them two now. Loren asks what about the boy? Katherine is pregnant with Doug's baby after one night of him drugging her and raping her.
It took me awhile to figure out why this was important. Then it came to me that her unborn child is her second child. Will it be evil? Maybe a sequel will explain?