Nagamine Tomoki tries to confess his feelings to the upperclassman he most admires: Ayase Sayuki, but he hesitates by seeing a deeply sad expression on her face. He continues to work alongside her in the school’s library committee, seeking to learn more about his loved one.
A certain day, he visits a seemingly abandoned bookstore, where he finds a mysterious locked book called “Your Diary”. Misuzu – The owner of the place – says the book chose him and gives him the key.
When he unlocks the diary, a cute little girl comes out of it and introduces herself as Yua, the goddess of happiness. She explains that her illustrated diary has become Tomoki’s property and it will fill itself with new content for each of Tomoki’s new happy memories.