A Woman.

Satoru has been living with her girlfriend Misa for a while, and is considering proposing to her. One day he shows her photo to two of his old friends, and they both say they were also dating four years ago – at the same time, but in completely different cities. She looked the same, hummed the same song, and had the exact same name, but then suddenly disappeared one day. Satoru starts to suspect that Misa is hiding something from him.