A Promise to 108P in a Year summary: A college student named Mamoru Shiodome, inexperienced in love, one day saves an old man who mysteriously grants him a strange mission. To form deep emotional connections with 107 people within a year to prevent the end of the world. As he tries to understand this bizarre task, he suddenly begins to see signs of hidden desires in those around him, even in a top idol, Haruka Kiyose. Can Mamoru help others confront their emotions and save the world through love?