Our hero is Teppei, a boy who is changing schools due to his beloved mother's unfortunate circumstances.
The teacher in charge of Teppei's new class is called Rikako.

Teppei sees his mother in Rikako.
Rikako realizes what's on the boy's mind, and tells him to come see her after school for a "special" course. An obscene sexual education ensues.

[from DLSite English]