Masashi Kusakabe, portrayed by Takuto Sueoka, begins a new chapter in his life as he relocates to a charming suburban area with his mother, Yasuko, played by Mayu Tsuruta. Leaving behind the haunting memories of his deceased father, Masashi conceals his grief behind a mask of cheerful demeanor, but the shadows of his past loom large as they settle into their new home, which appears to harbor untold secrets.
Upon exploring his new surroundings, he encounters a curious creature that resembles a jellyfish and affectionately names it Kurage-bo (Jellyfish Boy). This whimsical being shares Masashi's fondness for chee-kama (cheese-and-fish-cake sticks) and possesses the unique ability to fly. In no time, they forge an inseparable bond that transcends spoken language, delighting in their newfound companionship.
As Masashi begins school with Kurage-bo hidden securely in his backpack, his anxiety about concealing their relationship grows. However, he soon realizes that his classmates are also accompanied by similarly peculiar creatures, known as F.R.I.E.N.D.s. These fantastical companions are generated and controlled using specialized Devices that have become an essential aspect of the children's daily lives. When a classmate, Tatsuya, challenges Masashi during class, Kurage-bo showcases its impressive kung-fu skills by triumphing over Yupi, Tatsuya's F.R.I.E.N.D., in a spectacular duel.
Upon returning home, he finds his uncle, Naoto, portrayed by Takumi Saitoh, engaged in a heated argument with his mother at the doorstep. “This town is treacherous,” Naoto warns Yasuko, who vehemently replies, “You want us out of here?” Later, Naoto instructs Masashi to report any peculiar happenings, but when he spies Kurage-bo in the backpack, he wisely chooses not to comment.
One day, while heading home from school, Masashi unexpectedly stumbles upon his peers embroiled in a fierce battle at a tranquil shrine. Pressured into joining their ranks, he is quickly surrounded by their F.R.I.E.N.D.s, causing panic when Kurage-bo disappears. Just as despair begins to settle in, Luxor, Saki's F.R.I.E.N.D., played by Himeka Asami, emerges alongside Kurage-bo, and together they fend off the impending threat, plunging Masashi's reality into a swirl of wonder and curiosity.
While confiding in Masashi, Saki divulges the origins of her Device, which was given to her during a tumultuous family period instigated by top-level members of the local research center. She describes how these cloaked figures presented themselves as heralds of hope, promising friends who would never betray her. Unknown to the children, this has sparked a chaotic phenomenon in the town, forcing them into battling their F.R.I.E.N.D.s.
As the annual school festival unfolds at the university, it contrasts sharply with the tensions flaring from a protest led by Saki's mother, Shizuko, portrayed by Asuka Kurosawa, who is part of a cult vehemently opposing the university's research. The situation reaches a boiling point, pushing Saki and Masashi to escape into the research center.
Meanwhile, the mysterious Black-Cloaked Four, drawn to Masashi's unique energy, devise a scheme to claim him. As Masashi and Saki share their inner battles, their connection deepens. However, Tatsuya and Juran, aware of Naoto's connection to the research facility, plot against Masashi and kidnap Kurage-bo. Their actions bring the attention of the school's gym teacher, who punishes the boys for their cruel behavior.
During a visit to Masashi, now recovering in the hospital from an encounter with Yupi, Naoto presents him with a Device purportedly “for protection.” This seemingly thoughtful gesture feels deeply unsettling amidst the chaos.
As news of Naoto's shocking demise circulates, Masashi grapples with his mother’s heartbreaking revelation while focusing his efforts on finding Kurage-bo. When Saki discovers Masashi, she urgently questions him regarding her malfunctioning device, leading them to consider the culpability of Tatsuya and Juran.
In a dramatic turn, the two confront Tatsuya and Juran, who are now allied with the enigmatic Cloak Four. Koh, an orphaned prodigy, reveals his anime-inspired alter ego, Ko2, initiating an epic skirmish against Yupi and Shimon that culminates in their downfall. Just as defeat threatens to engulf them, Masashi steps in, granting the Cloak Four the precious time needed to regroup.
After the dust settles, Tatsuya and Juran comfort Koh, essentially signaling that he won’t journey through trials alone. In a climactic twist, the true Naoto confronts Koh's duplicate, and the battle for understanding intensifies as secrets unravel.
As the clock winds down, Saki finds herself desperate to rescue her mother from the destructive forces at play, only to be saved by Masashi in a moment of dire need. With Naoto revealing the Oval's vulnerability, Masashi seizes the chance to eradicate this overwhelming threat once and for all. As the F.R.I.E.N.D.s vanish into oblivion, including Kurage-bo, a new chapter of possibilities begins to dawn.
In a poignant moment of sorrow, Masashi’s cries resonate through the desolate surroundings, his loss profoundly felt. Saki’s empathetic expression reflects their shared grief. Koh arrives with friends, and Tatsuya and Juran, remorseful for their earlier actions, strive to reconcile their mistakes. With renewed determination, Koh utilizes his phone to bring together the F.R.I.E.N.D.s, facilitating their return to a world they cherished. As Masashi, Saki, Kurage-bo, and their friends finally reunite, an air of resilience fills the atmosphere, paving the way for a brighter, more promising future.
