"Kyōkai no Kanata: Idol Saiban! - Mayoi Nagara mo Kimi o Sabaku Tami" is a comedic spin-off of the original "Kyōkai no Kanata" series, blending the supernatural elements of the main story with a lighthearted idol-themed narrative. The plot revolves around Mirai Kuriyama, Akihito Kanbara, and other members of the Spirit World Warriors club as they are unexpectedly thrust into the world of idol competitions. The story begins when the group is tasked with investigating a series of strange occurrences linked to a popular idol group. These incidents involve mysterious youmu, supernatural creatures that threaten both the idols and their fans.

As the Spirit World Warriors delve deeper, they discover that the idol group is being targeted by a rival faction seeking to sabotage their performances. To protect the idols and uncover the truth, Mirai, Akihito, and their friends decide to form their own idol group, competing directly in the same events. The narrative unfolds through a series of comedic and dramatic moments as the characters balance their supernatural duties with the challenges of idol life, including rigorous training, rivalries, and stage performances.

Key characters include Mirai Kuriyama, a spirit world warrior with the ability to manipulate blood as a weapon, and Akihito Kanbara, a half-youmu with regenerative abilities. Their dynamic, along with the quirky personalities of Mitsuki Nase, Hiroomi Nase, and Ayaka Shindou, adds depth to the story. The series explores themes of teamwork, perseverance, and the clash between the ordinary and supernatural worlds, all while maintaining a humorous tone. The idol competition serves as a backdrop for both comedic antics and moments of character growth, as the group learns to navigate the unfamiliar territory of the entertainment industry while confronting supernatural threats.