"Reunion" is a 12-episode anime series that blends elements of mystery, drama, and science fiction. The narrative centers on a group of six childhood friends who reunite after a decade apart, drawn together by a cryptic invitation to their hometown. The invitation, sent by an unknown sender, promises answers to a long-buried secret from their past. Upon arrival, the friends discover that their hometown has been abandoned, with no signs of life except for a single, futuristic facility at its center.

The facility, known as the "Chrono Nexus," is revealed to be a time-manipulation device created by a shadowy organization. The friends learn that they were unknowingly part of an experiment during their childhood, where their memories were altered and their lives intertwined with the Nexus. The device has the power to rewrite reality, but its misuse has caused temporal rifts, threatening to unravel the fabric of time. As the friends explore the facility, they uncover fragmented memories and clues about their shared past, including the disappearance of a seventh friend, whose fate is tied to the Nexus.

The main characters include Kaito, the de facto leader of the group, who struggles with guilt over his perceived failure to protect his friends in the past; Aya, a pragmatic and resourceful individual who becomes the group's strategist; Ren, a quiet and introspective artist haunted by visions of alternate timelines; Sora, a cheerful and optimistic figure who hides a deep sense of loss; Haru, a tech-savvy engineer with a penchant for solving puzzles; and Mio, a former prodigy whose connection to the Nexus runs deeper than anyone realizes. Each character's personal journey intertwines with the overarching mystery, as they confront their fears, regrets, and the consequences of their actions.

As the series progresses, the friends must navigate a series of challenges within the Nexus, including temporal anomalies, paradoxes, and encounters with alternate versions of themselves. The narrative explores themes of memory, identity, and the impact of choices, as the characters grapple with the question of whether they can—or should—change the past. The climax reveals the true purpose of the Nexus and the identity of the sender, culminating in a decision that will determine the fate of their reality. The series concludes with an open-ended resolution, leaving room for interpretation about the ultimate consequences of their actions.