From Seraphic Deviltry:

Munakata has a secret: right after caching a suspect and closing a case, he feels the unstoppable urge to release sexual frustration. As a way to alleviate his repressed desire, he sets up online videos where he shows how he gives himself pleasure. In one of those moments, Tooru, Munakata's loyal kouhai, hears some strange noises. Thinking that his beloved senpai might be in need of help, he bursts in right in the middle of an online transmission. From there on, everything gets complicated, but Tooru is sure of one thing: his tortured senpai needs his help and support, and maybe something else?