"I'll show you a doll."
To celebrate the 20th anniversary of their idol Mamiya Kazune, a group of people in their forties gather on Kazune Island where they once spend a year together worshipping her. Looking back on their young days they are accompanied by Kisaragi Uyuu and his assistant Maina Touri, who are supposed to interview them and write an article about their reunion. Eventually a decapitated corpse turns up... on a snowy morning in August.
Invited by their club's first female member Arima Maria, Egami and Arisu go on holiday on her uncle's solitary island resort. Her grandfather, from whom her uncle inherited the island, loved puzzles and also created one before his death, which lets you find his heritage in the form of diamonds once you solve it. While the students ponder on the meaning of the numerous Moai statues scattered over the whole island, 2 guests are found murdered in a locked room and the 2-way radio, the only way to inform the mainland, has been smashed up...
Various detectives are gathered on Erikajima to investigate Alice Mirror Castle and find the Alice Mirror. A chess board with 10 white figures makes the detectives aware of their situation being strikingly similar to a certain literary masterpiece and the further development unfortunately supports their hunches...
After being accused of murdering her parents at the age of 14 and considered as certifiably insane due to her multiple personality disorder, computer scientist and overall genius Magata Shiki retired from the world and lives in her own little locked room in the Magata research facility on an isolated island. 2 doors separate her from the rest of the facility. The inner one can only be opened with her admission while the outer door can only be opened by the other facility members and she has not left her space for 15 years. When engineering assistant professor Saikawa Souhei and his student and his former professor's daughter Nishinosono Moe request a meeting with professor Magata in this threefold locked room, a corpse wearing a wedding dress advances out of the room and what is left in there is a computer displaying the incomprehensible message 'all becomes F'...
Tsunojima, Aoyashiki's burnt down remains and a decagon-shaped guesthouse. The latter is used by seven students from a university's mystery club and as someone would naturally expect considering this setting, serial murders start happening on this otherwise uninhabited island with no one willing to confess the crimes. Two other students who did not visit the island with their fellow students receive mysterious letters from Nakamura Seji, the proprietor of former Aoyashiki and the whole island, who supposedly died in the incident in the mansion half a year ago. They find out that those letters were also sent to their meanwhile absent fellow students and they begin investigating the incident of Tsunojima's past...