The disease is called "Progressive Multi-Organ Neurodegeneration." It is a fictional intractable disease that appears only in this story and is different from real-world illnesses. The functions of the nervous system and multiple organs gradually decline. Initial symptoms include severe fatigue, fever, shortness of breath, dizziness, and loss of consciousness. As it progresses, assistance becomes necessary for walking, eating, and daily life. It may further affect breathing and swallowing. There is no established cure, and in most cases, the disease cannot be stopped. Life expectancy is stated to be around several months, but the speed of progression varies by individual, and the exact timing is unknown.

Guest and Leo Kuga are lovers living in the same apartment. Their relationship was going well, they loved each other deeply, and they envisioned a future after marriage. The story begins on the night Leo proposes to Guest. The next morning, Guest suddenly collapses and is rushed to the hospital. Although her life is saved, detailed tests reveal an extremely rare progressive intractable disease.