In a world where every person has a fixed lifespan visible only to a select few, high school student Kazuki Hasegawa is born with the ability to see the exact number of days a person has left to live. He keeps this power a secret until he notices that the quiet and reclusive girl sitting next to him, Yuki Shinohara, has a counter that reads zero. Despite the number indicating she should already be dead, she is very much alive and breathing. This paradox forces Kazuki to confront a hidden truth about reality: the numbers are not absolute predictions but rather measurements of how many words a person has left to speak.

The setting shifts between a melancholic modern Tokyo and the ethereal Library of Spoken Echoes, a liminal space that exists between life and death. In this library, every word a person will ever utter is written in a book. Kazuki discovers that he possesses a forbidden ability to rewrite the words in someone else's book, effectively extending their life by giving them new words to speak. However, each word he writes is stolen from his own remaining count. The core conflict emerges when Yuki reveals that her book has been empty for years, meaning she has been living in complete silence, believing that speaking a single word would erase her from existence.

The main characters include Kazuki, a pragmatic youth burdened by a power he never wanted; Yuki, a girl who communicates through handwritten notes and subtle gestures, possessing a gentle wisdom that belies her tragic circumstances; and Ren Amamiya, Kazukis childhood friend who is slowly dying from a terminal illness, with only a few hundred words left to live. Ren serves as both a rival and a tragic foil, as he also possesses a weaker version of the word-manipulation ability and believes that letting people die naturally is the only ethical choice.

Notable narrative arcs include the Silent Promise arc, where Kazuki secretly writes a single word into Yukis book to prove his theory, inadvertently causing her to speak and shattering her silent curse. This act binds their fates together. The Word Thief arc follows a mysterious antagonist who steals words from others to achieve immortality, forcing Kazuki to choose between saving a citywide crisis or preserving his own voice for Yuki. The final arc, The Last Thousand, sees Kazukis counter drop to exactly one thousand words. He must navigate a labyrinth of emotional confessions, final goodbyes, and a climactic choice: write a new life for Yuki at the cost of his own final breath, or speak the words he has always wanted to tell her and accept the natural end of both their stories. The narrative explores whether a life measured in words is any less meaningful than one measured in years.