Strange and horrible things are happening in the city, right under our noses, and most of us never find out. Those who do, often die. Four high school girls encountered the horror, and died. That's not what this anime is about. It's about what happened to them afterwards. Other reviewers have given a good plot outline, so I will content with what I liked and didn't like.
First, overall, I liked this. It's above average in concept, in character development, in dialogue. I own it and plan to watch it again. I plan to give it to my friends. It features a group of sane girls trying to cope with insane personal disaster. It's their story, and so we are spared the clueless, gormless, constantly embarrassed male co-star, or the loud and bratty little sister, or the overly moe pet. It's surprisingly quiet. Yes, there's too much shouting during the fight scenes, and some of the secondary characters can be accused of overacting, but it's not one of those anime where everyone teeters on the brink of adrenalin exhaustion. Each of the characters is a unique individual, and is believable, even when their circumstances aren't. Rose, for example, fighting to keep her family together and care for her younger siblings, with her mother in the hospital and her father run off, and no sign of NYC Family Services.
SPOILER ALERT, all the way down.
That said, there's plenty of plot holes, and your disbelief has to be hanged, not just suspended. We never find out why the girls were enticed to the old house in the first place, nor why they were killed, or why they were left. Was it all a plot to get one New Body? If they were recruited to fight the other clan, why weren't they trained to fight? Why weren't they given better (i.e. any) weapons? Then there's the pharmaceutical company, with their inept experiments. If the drug side of the operation is run the same way as their attempt to preserve a breeding stock, it's a wonder we're not all dead. What's the purpose of the red flowers and the butterflies? Wouldn't a cell phone do as well? Even a pager. What happened to the seed the girls planted at the cemetery? Why do the old New Bodied women die and dust when the curse is lifted, but Our Girls just fall asleep, and wake up immortal but without memories. These kind of gaps run all through the program. If you are the kind of person who can just say "magic" and ignore them, you will like the series. If not, you will have trouble not shouting at the screen.
In the end, the girls come to terms with their future, understand that they will be remembered (even though they won't remember), and win through to a hollow victory. Everybody dies. On both sides. Except them. The school and its island are covered in a Red Garden of flowers, where they awaken. That's when the fun begins.
For me, the post-season OVA (original video animation) was the best part. It takes place hundreds of years in the future. There are flying cars. The supporting characters are all future versions of the ones from the series (the two policemen are still on patrol, the same guy runs the hamburger stand). We have just spent 22 episodes establishing the characters, and now we can have fun playing off of them. They are all still biologically 17, even though they are chronologically (and emotionally) much older. Rose is more assertive. Rachel is a bit of a slob (showers are so much trouble). Claire is still driving her old car. They make bets on who can seduce the new transfer student. They fight crime while singing (extremely poorly) a song set to the music that plays for the setup screen. Claire builds a sexy female gundam style mech, one that ends up face down in the flowers of the Red Garden, with its bare backside in the air. Kate, less changed than the others, manages to shock them all at the very end, and I think I'll keep that spoiler to myself.