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It all starts with a hurricane hitting the town of Homestead, Florida. While everyone else is preparing for the storm, a young girl named Rose Varon goes out looking for her cat. Then her father, park ranger Russell Varon, goes out looking for her. Separately, his ex-wife, Dr. Mariel Underlay, also goes looking for Rose. Russell finds the girl, but their vehicle gets overturned, and they spend the night trapped inside, waiting out the storm. The cat, btw, was in the truck the whole time. In the morning, it seems Mariel is missing, so folks go out searching for her, and find her floating in a marsh. Coincidentally, during the storm, Rose had seen some strange lights in the sky out over the marsh. At first the only one to take her seriously is Dave Groves (Russell's brother-in-law). He's the brother of Larkin, Russell's new wife, an investigative reporter, who's expecting a child with Russell. Dave is kinda funny, weird, something of a conspiracy nut, and keeps a blog about all the strange stuff going on in town (which you could actually read while the show was airing, though the site is now gone). Other characters include Mariel's new husband, Sheriff Tom Underlay; Rose's old brother Jesse Varon, son of Russell and Mariel; and Kira Underlay, Tom's daughter from a previous marriage.
Anyway, after the storm, Mariel starts acting strangely, and Rose is the first to notice something different about her. Although it seems Tom knows what's really going on. There's actually a lot going on, it seems, but the show really takes awhile to build up to it all. For quite awhile no one really takes Dave's concerns about a possible alien invasion seriously, and even once Russell realizes something's going on, he's not ready to accept aliens as an explanation. But there do turn out to be weird, phosphorescent creatures in the water. And it turns out lots of people have been... changed, by them. As I said, Tom knows what's going on, has for like 10 years... but he doesn't know everything, and it turns out he can't trust everyone he thinks he can. One person in particular, a man named Szura, becomes a major villain. And... well, it's not just people who have been changed who know what's going on, and what the main characters are only slowly figuring out.
But, I'd rather not say any more, don't want to spoil too much. Though I will say I sort of want to put this under "supernatural & paranormal," but for now I think "mysterious" is a slightly better fit. Anyway, further into the season, the pace of the show really picked up, things started getting much more interesting. Unfortunately, it didn't get a second season, so a lot of stuff was left hanging....