Dr. Ramune -Mysterious Disease Specialist- S01E09 - Mysterious Labyrinth

Upon receiving an invitation to a treasure hunt from the famous mysterious item artist Reinette, Ramune and Kuro visit Reinette's mansion. There they encounter Ayame and Nico, who have also come to participate in the treasure hunt. However, when Ramune sees the man with them, his expression freezes, and he attempts to flee the scene. That man is Ramune's master Momiji!