Ryakudatsu Sareta Fiancee ~Koibito Masumi to Otouto no Himitsu

A few years after leaving his hometown, the protagonist Yuto reunites with his childhood friend Masumi and they start dating, and later get engaged. They are
now living together in their new home and are living happily.

One day, he receives a call from his younger brother Hayato, who he has been somewhat estranged from recently because he now lives in the countryside, saying that he has come to Tokyo. When he hears the situation, he learns that his company went bankrupt, so he stays in a business hotel and comes here to look for work.

Wanting to help his younger brother, Yuto invites him to stay at his house for a while. Although his younger brother refuses, saying that it is wrong to stay at a place where his fiancee is living together, Masumi convinces him that he need not feel uncomfortable because he is also his childhood friend, and takes him to his house.

However, contrary to his expectations, Masumi seems confused about his younger brother staying over. However, after learning the situation, she accepts the proposal, and they start living together with their younger brother. Yuto remembers the times when the three of them used to play together in the past, and wants to continue to get along well with his younger brother.

However, what Hayato and Masumi were doing without Yuto's knowledge...
Soon, Yuto would find out in a shocking way.