It's a freak plant-type urban legend that turns out to be a bit too real. It indoctrinates people and the manga mainly revolves around the MC feeding it.
Now, I will say that the plot never really gets that interesting and in general way too little development actually happens. It reads a lot like a one-shot, where the mangaka just had a cool idea he wanted to see happen and then just did it. Very little regard seems to have been made to the characters, who are pretty plain and boring.
If you're reading this, you're reading it for the premise and the art, which in my opinion are well worth it, but your mileage may vary.
