"Aikotoba", a popular matchmaking app with a feature of "finding your fated partner".

The protagonist, Mizukami Takumi, is an office worker with a stable job. One day, he is recommended by his colleague to try the app. Through it, he meets a girl called “Yozora” with compatibility of 99% and gradually become closer each day.

Coincidentally, Takumi is being sent for business trip to Sendai, the city of where the girl is. As a result, they decide to meet up in real life and arrange a meeting there.

However, when the time comes, he wouldn’t believe what he sees.

“…Onii…chan?”

Under the cold sky of Sendai, Takumi didn’t expect to meet “her”… his younger sister, Uzuki, who he hasn’t see in seven years. Moreover, she is the girl under the alias “Yozora” all this time, the girl in the app who made him fall in love.