Terutoshi awakens a spirit when he kills a baby bird on the way to school. To right the wrong and fulfill its dream of growing up and bearing offspring, the spirit curses Terutoshi with a task: to impregnating another person. Branded with the spirit's seal, his best and only choice now is to fornicate with Ms. Sayumi, his teacher.

But there is a special effect of the curse: when Terutoshi thinks of any erotic act, he becomes transparent!!
To regain opacity he must travel the campus unseen to target Sayumi, having sex with her during class, during breaktime, and so on!

[From DLsite English]