Enrolled in a certain elementary school is an unpleasant 11-year-old boy with a highly cynical outlook on society. Believing that the world lacks individuality, he wants nothing more than to be unique and remembered. The boy seemingly gets the chance to leave his name in history when he becomes the first to be diagnosed with a new incurable disease that is estimated to kill him by the age of 12.

Just as he is rejoicing about his newfound individuality, he meets a cheerful, prodigious girl who has the same disease—and is nearing her 12th birthday. As the boy realizes that he won't be the first to die from the disease and therefore won't be remembered, he forms a twisted plan: kill the girl before her illness does.

(Source: MAL Rewrite)