Me, the Cat and the 20-year-old You

Rintaro Waga, an old writer who has spent his whole life alone, is confronted by a boy who claims to be his best friend Shiyou, who is supposed to be dead. Although he doubts whether he is the real Shiyou, Rintaro decides to spend some time with him. Will they fulfill that innocent promise they made 40 years ago ? (Source: Coamix, translated)