"Happa 64" is a manga series that explores themes of identity, memory, and the passage of time through a nonlinear narrative structure. The story revolves around a group of individuals connected by a mysterious incident that occurred in their childhood, centering on a peculiar game they played called "Happa 64." This game, which involved collecting leaves and assigning them numerical values, becomes a pivotal element that binds their past and present.

The protagonist, a young man named Haruto, returns to his hometown after years of absence, prompted by fragmented memories of the game and its significance. As he reconnects with his childhood friends—Yui, a reserved artist; Kenta, a pragmatic mechanic; and Aoi, a free-spirited wanderer—they collectively begin to unravel the enigma of "Happa 64." The game, initially perceived as a simple childhood pastime, gradually reveals itself to be a complex system with cryptic rules and hidden meanings, tied to a tragic event that shaped their lives.

Through alternating timelines, the narrative delves into the characters' shared past, exploring their interactions, misunderstandings, and the emotional scars left by the incident. The game's mechanics, which involve assigning numerical values to leaves based on their shape, color, and texture, serve as a metaphor for the characters' attempts to quantify and make sense of their experiences. As the group pieces together the fragments of their memories, they confront unresolved guilt, regret, and the lingering impact of their choices.

The story also introduces a secondary layer involving a mysterious figure known only as "The Gardener," who appears to have orchestrated the game and its consequences. The Gardener's motives remain ambiguous, adding an element of suspense and intrigue as the characters uncover the truth behind their shared history. The interplay between the past and present, coupled with the gradual revelation of the game's true purpose, drives the narrative forward, culminating in a resolution that ties together the characters' emotional arcs and the central mystery of "Happa 64."

The manga's exploration of memory and its subjective nature is underscored by its intricate plot structure and the characters' evolving relationships. Each character's perspective contributes to the overarching narrative, highlighting the ways in which their interpretations of the past diverge and converge. The series balances its introspective themes with moments of tension and discovery, creating a compelling and thought-provoking story.