Tarou Tsudahayashi has always liked Umi Kanzaki, his childhood friend. The first time he confessed, they were in kindergarten, and she responded that she liked beetles. In grade school, she misheard him, and in middle school, she misunderstood him once again. But the fourth time's the charm, and in high school, Umi finally gives a favorable response.

However, it is at this moment that Tarou realizes Umi has less than one hundred days to live. Ever since he was young, Tarou has been able to see the remaining lifespan of living creatures close to death. He has never been able to extend the time, but it turns out that Umi is an exception: when her heart skips a beat, the number of days goes up!

Kimi ga Shinu made Ato 100-nichi follows two childhood friends as they try to avert death, all the while learning the meaning of love.