A small mouse, a cheerful and loyal friend, is living his life as usual, unaware that his remaining time is already being counted down from one hundred days. This is the premise of 100-Nichi Go ni Shinu (X) Nezumi, a sequel that shifts focus from the original story's crocodile protagonist to one of his closest companions. The narrative follows the mouse, named Nezumi, through what appears to be an ordinary stretch of days, but the reader is always aware of the dwindling number. This knowledge casts a profound sense of poignancy over every scene, whether he is sharing a meal, going about his daily routine, or laughing with friends.

The story is set in a familiar, gently anthropomorphic version of modern Japan, where animal characters live, work, and socialize much like humans. Nezumi is the central figure, and his perspective is the lens for the entire work. Other characters from the original story return, notably the kind-hearted crocodile, Wani, who is shown in flashbacks or memories, his absence being a defining emotional undercurrent. New and existing acquaintances populate Nezumi's world, including his best friend, the energetic and loyal dog, Inu, and others like Kaeru the frog and Mogura the mole. The narrative structure is deliberately simple and serialized, with each short installment, typically a four-panel comic, representing a single, unremarkable day in the countdown.

While the original work depicted the crocodile's final hundred days, leading to a sudden and tragic death, the sequel explores the aftermath of that loss and the experience of living with that knowledge from a new vantage point. The notable narrative arc is the countdown itself, which creates a quiet but relentless tension. Unlike the first story where the audience discovered the premise alongside the crocodile's oblivious days, this sequel carries the weight of the original's ending. The audience knows the rules of this world: the countdown is absolute. The arc follows Nezumi as he navigates his ordinary life with a hidden expiration date, his interactions taking on a deeper meaning as the days tick by. The story builds towards its inevitable conclusion, a final day on which the promised event occurs, echoing the format and emotional impact of its predecessor.