Three years ago, a disappearance case, known as the "Joker Case," was suddenly closed without being solved. Ichiro Kunishita, an investigator from the Public Security Bureau of the Tokyo Metropolitan Police Department who was called for a reunion of the investigation, is ordered to take on a completely different mission. He is assigned to spy on and monitor Shiro Hanabusa, the young head of a designated gang clan. He is said to be a man of many mysteries who has risen to the rank of Wakagashira (young leading subordinate in yakuza group) since he joined the gang three years ago. In the course of his mission to get close to Shiro, Ichirou learns a shocking fact, and decides to make a fake "lover's contract" with him. Their secret investigation into the case gradually leads them to an incident that will shake the nation and their destiny. Adapted from the manga "Kei×Yaku: Abunai Aibou" (ケイ×ヤク－あぶない相棒－) by Kaoruhara Yoshie.