The Idol Stars' Secret (Part One) Meitantei Conan : Season 9 Episode 30

Kogoro is invited to Yoko's engagement--no, Yoko's friend's engagement with a famous actor. When Kogoro arrives, though, he is hit with a tennis racket by mistake! It turned out that Yoko's frind had been blackmailed by an unknown person, and had been very careful when opening doors since. While everyone is watching a video, Yoko's friend goes showering. After quite a time Conan realises that something must be wrong and goes into the bathroom to find Yoko's friend lying in the bathtub with her neck cut.