This is one of the most mysterious anime that I have ever seen.
It is a famous murder mystery series that was adapted from the novel of the same name, which was produced by 07th Expansion. Aired from April 2006 to December 2007, with some OVAs later released, this series plot starts in 1983, with the story of Maebara Keiichi, a transfer student that arrives at the peaceful village of Hinamizawa (a fictional rural village based on the actual Shirakawa in Gifu, a World Heritage Site).
He becomes friends with a group of girls, who are his classmates. At school, he participates in some club activities that consist mostly in card and board games, as well as hide-and-seek.
Hinamizawa appears to be a normal, peaceful, rural village to Keiichi. However, the tranquillity abruptly ends after the annual Watanagashi Festival, a celebration to commemorate and give thanks to the local god, Oyashiro. Keiichi learns that every year for the past four years, one person has been murdered and another has gone missing on the day of the Watanagashi Festival. Keiichi himself soon becomes drawn into the strange events surrounding the Watanagashi Festival and Oyashiro.
I thought about showing a video with scenes from the anime, but showing spoilers in this case might be a “crime”, so I prefer to show you this creepy opening, and let you imagine how it actually is 🙂