1x11 - The Dinner Table Detective

Reiko attends the wedding of Sawamura Yuri, who she cherishes like a younger sister. The groom is Hosoyama, a consultant for a family friend. When Yuri is attacked and gravely injured in her room during the party, Reiko risks her identity being revealed to help solve the case. But when the crime scene is found to be a perfect locked room, Reiko is suspected of being an accomplice.