Akira Ran, an office lady in her thirties who dreamed of a perfect marriage, loses both her fiancé and her dreams when he betrays her. She reunites with her former high school softball teammates: Homura, a twice-divorced woman who is unexpectedly diagnosed with only one year left to live, and Kei, a married friend who also feels her life has stalled. Realizing that ten years after graduation none of them have achieved their teenage ambitions, the three women decide to take drastic action. To fulfill Homura’s hidden dream of running a detective agency, they quit their jobs, move in together, and open a small agency in the Asakusa district of Tokyo.

As amateur detectives, Ran, Homura, and Kei tackle a variety of cases—from petty disputes over children’s toys to serious land‑grabbing schemes—while navigating their own messy personal lives. Central to the story is Homura’s decade‑long search for her first love, a man named Tsukishima, whose sudden disappearance is linked to the mysterious “Golden Dragon Legend.” Meanwhile, Ran obsessively pursues the truth behind the “diamond” that gave the series its title—a symbol of both lost hopes and the possibility of a fresh start. The narrative weaves together comedy, mystery, and heartfelt drama as the three friends support each other through Homura’s limited time, confront their past regrets, and attempt to engineer a life‑turning reversal before it is too late. Notable story arcs include the “Goriyama” incident and the eventual return of Tsukishima, which forces the reunited quartet to face the full weight of their shared history.