"Energy conservationist" Hotaro Oreki joins the Classic Lit Club at his high school at his sister's behest, to keep it from being shut down. Also joining the club are his best friend, Satoshi Fukube, and Eru Chitanda, a curious girl who enjoys mysteries.

Eru grows bored of the club's lack of activities, and suggests that they prepare an anthology to submit for the school's culture festival in the fall. While tracking down previous anthologies, the club discovers a mystery in the school's library.

Over coffee, Eru opens up to Oreki to explain why she joined the club, and asks him for his help with a personal matter. Still unable to locate any anthologies, the club members run into an obstacle when trying to search the club's previous room.

Eru decides to solicit Satoshi and Mayaka's help hypothesizing what happened to her uncle. The club members meet at the Chitanda estate, where they present their research and surmise what they believe happened 45 years ago.

Hotaro is torn between continuing with his "gray" life and the "rosy" life that the others live. A phone call from Hotaro's sister leads him to believe that there are pieces missing to his theory regarding the events of Sekitani's expulsion.

Hotaro's history class is interrupted by an outburst coming from Eru's math class, prompting a discussion in the Classic Lit Club about the nature of the seven deadly sins, and what might provoke Eru to become angry.