Yuuki Unabara is a college student dating high schooler Sayako Amanokawa, creating an age gap. Their relationship falters due to Yuuki's secrecy about his membership in TSI, a global organization spreading Buddha's teachings and combating demonic forces. He maintains this concealment throughout their romance, causing significant tension until a temporary breakup occurs.

To reconcile, Yuuki arranges a meeting between Sayako and Mari Kimura, a defector from the rival Sounen Group to TSI. During this encounter, he reveals his TSI affiliation. Using a poorly drawn diagram on a napkin, he explains the Spirit World and the nature of suicidal souls. Sayako criticizes his handwriting and drawing skills, which he defensively dismisses.

Yuuki assists Sayako on her spiritual journey. He intervenes when her younger brother, Shunta, sneaks into Sounen Group headquarters, extracting both siblings from danger. His involvement proves crucial when Shunta falls ill from demonic influence; Yuuki brings him to a hospital and supports the family during the crisis.

The couple reconciles after Sayako joins TSI and becomes a follower of Master Sorano, Buddha's true reincarnation. She moves in with Yuuki. As a TSI member, Yuuki works to expose Tousaku Arai, the false Buddha leading the Sounen Group, and combats supernatural threats like an illusory UFO attack at a summer festival. His role focuses on supporting TSI's mission to disseminate spiritual teachings and counter dark forces.