Mars loses his twin sister at the beginning of the book in a horrific accident. She's definitely not herself when it happens. When some friends from the summer camp she went to arrive, Mars is intrigued by them--and also by the bees that seem to hang around them. He decides to go back to the camp--the one he left when it became clear they couldn't deal with anything that wasn't part of the gender binary. Camp hasn't improved much, but Mars is determined to find answers about his sister. 

The writing has a dreamy, honey soaked quality to it that makes you think of dense summer days in the forest. You can feel early on that something is happening, but it is revealed to you slowly. I haven't watched Midsommar, but I suspect they follow similar horror vibes. Bees factor heavily in this story.