Hirosaka Kouta lives together with her younger sister, Kaede, due to their parents' jobs. He is an honor student with excellent grades at school. However, he is a severe siscon and loves Kaede from the bottom of his heart.

One day, He accidentally drops his phone in a plant when coming home from school.
When he tries to pick it up, he gets bited by an unknown being, presumably an insect.
However, the wound soon disappears as if nothing had happened.

When he gets home, his arms suddenly become tentacles and he rapes his sister involuntarily! They also find out that his arms gets back to normal after he had sex with her.
As a result, Kaede offers to 'help' him whenever his arms turn into tentacles.

What will happen to their life after this?

[Based on the official website's description]