Eri Shingai presides as class president at St. Arcadia Academy and founded the Treasure Hunting Club. Her significant personal wealth allows her to bankroll the club's expeditions, including providing a private cruiser for an oceanographic mission. She recruits licensed salvager Aika Sumeragi to investigate a butterfly insignia tattooed on Karen Minamino's left breast, discovering it encodes coordinates to an underwater site.

Shingai exhibits persistent curiosity and strong organizational leadership during the expedition, coordinating activities despite sabotage attempts by their school advisor, Risako Nagisa. Her interactions reveal a bossy streak, often mocking classmates' academic shortcomings, yet her fundamentally kind personality surfaces through genuine concern for Karen's safety as the mystery unfolds.

The discovery of an ancient underwater airport and biogenetic technology linked directly to Karen's tattoo drives Shingai's involvement. She actively pursues Karen's forgotten past connected to experimental research, maintaining her role as the expedition's primary facilitator. She leverages her resources to advance the investigation into the tattoo's significance and its ties to advanced technology.