Rokudo is a school city that gathers all the necessary facilities on one island, with Rokudo Academy at its center.
There is no cell phone reception and no internet. Everyone lives in dormitories provided by Rokudo.
While it feels like it's been left behind by modern times, it has a laid-back atmosphere that you won't find in the city. However, it's a slightly unusual island with various rules.

Our protagonist, Hiiragi Seiichi, is a male student who transfers to the school at the invitation of the school principal.
He transfers at the busy time, one week before the cultural festival, the Rokudo Festival.
Classes, clubs, and affiliations are preparing for the festival.
Amidst all this, Seiichi interacts with various girls and deepens his friendship with them.
However, on the day of the festival, a murder occurs.
Seiichi's classmate is killed, and the culprit has yet to be found.
Several "potential suspects," including Seiichi, are gathered. Potential suspects are people who are believed to be involved in the murders for various reasons, such as motive and alibi.
In front of this assembled group of potential suspects, the school principal explains the case.
"If you weren't involved in the murder, please cooperate. But what would happen if you were the 'real suspect'? You don't want to think about it, do you?"

A closed, isolated island, with no way to escape, potential suspects and everyone else.
If their interests align, they can cooperate. But that doesn't necessarily mean they have to be on equal footing.
Testifying to alibis, denying motives. Potential suspects are hungry for "innocence."
So they keep quiet to avoid suspicion. They have no choice but to agree to unfavorable negotiations in order to prove their innocence.

[From Getchu]