Masumi and Toma are officially aunt and nephew.

Masumi took in Toma after he lost his parents 10 years ago, and they have been living together ever since.
They have a delicate relationship that is neither father and son, nor brother and sister.

Toma has had feelings for Masumi for over 10 years.
However, one night,
When he goes to pick up Masumi as she returns home on the last train, he witnesses a certain scene.

It is Masumi being touched by a pervert on the train.