Natsu e no Tobira

Natsu e no Tobira is a Japanese manga series written and illustrated by Keiko Takemiya. It is an early example of a shōnen-ai manga. It was originally serialized in Hana to Yume in 1975, published by Hakusensha, and it was re-released in 2000 by Kodansha. An animated film adaptation was released on March 20, 1981, in Japan.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.

Poster © the film's rights holders.