The episode begins with Totomaru visiting Kamonohashi, where he learns about his past. Kamonohashi was once one of the top students at BLUE, a prestigious detective school, but was expelled due to his "fatal flaw" of driving suspects to their deaths. Kamonohashi offers to bring cases to Totomaru and guide him behind the scenes.

Totomaru receives a low-priority case involving missing money from a piggy bank. Intrigued, Kamonohashi disguises himself and accompanies Totomaru to the home of two sisters. The younger sister claims that money has disappeared from her piggy bank, which is designed so that it cannot be opened without breaking it. Kamonohashi investigates the incident but initially declares the case unsolvable.

Meanwhile, Amamiya, a senior investigator, is looking into a murder case near a riverbank. She warns Totomaru not to interfere but becomes curious when Kamonohashi appears.

Kamonohashi returns to the piggy bank case and makes a breakthrough. He suspects that the piggy bank was broken and replaced with a new one and that the money inside was not stolen but replaced. He connects this to the murder case Amamiya is investigating: the younger sister killed her stalker with the piggy bank, broke it in the process, and then replaced it to hide the evidence. The sister confesses, confirming Kamonohashi's conclusions.

However, Kamonohashi's "fatal flaw" nearly resurfaces as he pushes the sister toward a deadly decision. Totomaru intervenes and prevents another tragedy.

The episode ends with BLUE staff discussing Kamonohashi's activities. They decide to investigate him, fearing that his involvement in cases could lead to more deaths.