Kimi no Saigo no Ikkagetsu, Koi wa Opal-iro ni natte

In the summer of their second year of high school, Tomoya and his childhood friend Ayano set out on a journey without a set destination, sharing their painful secrets as they search for "true happiness." As the "black lump disease," in which people die and turn into black lumps, spreads on the internet, Tomoya Sakuma, a second-year high school student, begins to connect with his childhood friend Ayano Haruhara after his mother's suicide. Though it takes him a long time to confess his feelings, Ayano becomes his beloved girlfriend. However, their time together is short. When summer break arrives, they decide to embark on a slow train journey with no set destination, filling it with things Ayano wants to do. But Tomoya doesn't yet know the truth that awaits him ahead..." (Source: Futami Sara Bunko, translated)