Masashi Kusakabe moves to a quiet suburban town with his mother, Yasuko, after the loss of his father. He wears a careful smile to light up their new life and to shield his mother from his own sadness, but a strange, almost unseen presence lingers in the apartment he carries boxes into. The first day in their new home starts with a gentle unease that grows into a curious bond when a mysterious creature slips into his world.
The next day, Masashi meets a creature that looks like a jellyfish. He names it Kurage-bo (Jellyfish Boy), an adorable being that adores chee-kama (cheese-and-fish-cake sticks) and darts freely through the air. Despite the lack of a common language, the two quickly become inseparable friends, sharing a quiet understanding that transcends words.
Masashi carries Kurage-bo into his new classroom, hoping to keep his friend a secret. But the moment the teacher’s attention shifts, the classroom erupts with a startling reality: every student has a similar companion, each paired with a small device that can generate and control these beings. The kids refer to them as F.R.I.E.N.D.s, and the devices in their hands allow them to summon and command their partners. A classmate named Tatsuya challenges Masashi, but Kurage-bo comes to his aid, defeating Tatsuya’s F.R.I.E.N.D, Yupi, with a display of surprising kung-fu.
Back home, Masashi encounters his uncle Naoto, who works at a local research center, arguing with Yasuko about the town’s safety. Naoto worries aloud, yet Masashi promises to alert him if anything strange occurs. Naoto notices Kurage-bo tucked in Masashi’s backpack but keeps silent for now, choosing to observe.
On a trip home from school, Masashi witnesses classmates playing around a shrine where their F.R.I.E.N.D.s battle each other. The mood quickly sours as the group pressures Masashi to join in, but Kurage-bo is missing, leaving Masashi exposed. A dramatic arrival follows: a powerful F.R.I.E.N.D. named Luxor emerges with Kurage-bo by Masashi’s side, and Masashi learns Luxor belongs to his classmate Saki. Saki reveals a troubling truth: her Device arrived during a moment of family strain, given by a shadowy cluster of researchers who promised “a friend who will never betray.” Four cloaked figures whispered their creed, and soon every child in town bears a Device. The battles become widespread, popping up all over the place.
A university festival at the research center becomes a focal point for tension. The cult to which Shizuko, Saki’s mother, belongs declares the research evil, staging a protest and dragging Masashi and Saki into the center’s orbit. As the Black-Cloaked Four plot to lure Masashi into their grasp, the two friends run away hand-in-hand into a world of danger and intrigue, where the energy around Masashi draws their attention and fear in equal measure.
In the wake of these revelations, Masashi and Saki grow closer, trusting each other with their fears and doubts. But trouble deepens as Tatsuya and Juran learn that Naoto works at the research center and plot to capture Kurage-bo. The gym teacher intervenes, punishing Tatsuya and his crew for harming Masashi and Saki, who are left dazed by the blow that lands when help would have saved them. Naoto, visiting Masashi, offers a Device “as protection,” yet something about it feels off.
Masashi’s world shatters when Naoto falls from a building, and the forest search for Kurage-bo yields silence. In a junkyard locker, Kurage-bo has been beaten and hidden away. Saki finds him, and Masashi admits that the device didn’t work and that Kurage-bo is missing. Together they push toward the lab, where a new confrontation awaits. There, Tatsuya and Juran confront Koh, who calls upon Ko2 (an anime-inspired ally) to battle Yupi and Shimon. Koh’s defense is fierce, and Masashi’s intervention helps push back the attack, even as a duplicated Naoto appears to complicate the fight. The real Naoto arrives in time to confront the counterfeit, and the truth behind the Black Cloaked Four’s plots starts to surface.
The cloaked operatives reveal their motive: they want to be reborn through destruction, using the children’s battles to harvest negative energy. Naoto chides them for their madness, and the plan to trap Masashi is laid bare. Realizing the risk to everyone, Koh stays behind to seek the Oval’s weak spot, while the others regroup to call every F.R.I.E.N.D. outside to join the last stand.
Saki makes a desperate attempt to rescue her mother but is pulled into danger herself, clinging to a railing as the Oval advances. Masashi catches her and pulls her to safety, and Naoto explains that the Oval’s weakness lies in the glowing orb on its head. With a final burst of teamwork, Masashi strikes the orb, and the Oval collapses. One by one, the F.R.I.E.N.D.s fade into a swirling vortex, pulling Kurage-bo back with them.
Grief and loss linger as Masashi mourns the absence of his floating friend, and Saki grieves alongside him, acknowledging the closeness they shared. Yet there is a glimmer of hope: Koh reunites friends, Tatsuya and Juran apologize for their earlier misdeeds, and Koh uses his power to restore every F.R.I.E.N.D. to the real world. In the end, Masashi, Saki, Kurage-bo, and the others begin to embrace a future colored by friendship and positive possibilities, united in their renewed sense of community and hope.
