桃尻インストラクター寝取りレッスン ～競泳水着でえろざんまい～

The protagonist is overweight so he decides to join a swimming club in neighborhood for moderate exercises every week. 

On the way home after the first day, he remembers he left his trunks behind and goes back to get it.

There, he witness an incredible sight.

The beautiful instructor he just met today is masturbating in the locker room using his trunks !?

In panic, he tries to return before being found out and tumbles into the room ...