On her birthday, Miyu receives a message left by her long-distance boyfriend. Following the instructions in the message, she searches for the diary in her room and begins solving the puzzles it contains. The clues he left gradually lead her back to the places where the two of them once shared cherished memories. At first, Miyu believes this is a carefully planned birthday treasure hunt.

As the clues unfold, Miyu starts to feel an inexplicable unease during their online conversations, causing her to question the true circumstances surrounding her boyfriend. When she finally solves all the puzzles and obtains the gift, all the clues and strange feelings come together, revealing the truth: her boyfriend has long passed away, and all her interactions have been with an AI generated from his data.

Faced with this reality, what choice will Miyu make? Will she immerse herself in the illusion or confront the truth?