Oni to Tengoku Sai

Tengoku, a school nurse, and Aoki, a high school teacher.
They have been lovers for a few months, and have been experiencing infatuation, jealousy, and little by little, they've begun to embody what it means to be in a "relationship".

One day, Aoki receives a phone call from a friend. He tells his friend that he has no intention of marrying his current lover. Tengoku, who was standing beside him, overhears him.

The next day is his day off, but he is about to leave early.
Aoki feels uncomfortable and decides to walk to the station to see them off. They kiss each other on an out-of-the-way street, but
Aoki is seen by a passerby and unintentionally mutters, "This is the worst". Aoki's expression changes and he leaves the scene alone.

Their relationship seemed to be going well, but...