Junpei Shiroyama is a high school student and gardening club member, dubbed the "Brain-blasting Genius" for his puzzle-solving talent. His childhood friendship with Ryusei Midorigaoka fractured when Junpei chose the gardening club over tennis in middle school.

Junpei discovers a mysterious rock that awakens as Spudio the 22nd, a dimensional messenger, transporting Junpei, three classmates, and their teacher to an alternate dimension. There, they face soul-forfeiting puzzle battles against Sphinxes.

Junpei consistently employs analytical thinking and leadership throughout these conflicts. He solves critical puzzles, like recalling the Doppler effect to defeat Sphinx Paramesos and rescue their teacher after an initial loss. He later aids Yurio in studying to rescue Tsuyoshi, whose existence was erased by a puzzle defeat, facilitating their reconciliation after Yurio confesses to causing Tsuyoshi's past accident.

Junpei's relationship with Ryusei evolves significantly. After Ryusei loses his "pride" to Sphinx Dictis and undergoes a personality shift, Junpei reveals Ryusei's past career-ending wrist injury. Solving both puzzles against Dictis restores Ryusei's demeanor and mends their friendship.

Junpei participates in confronting Sphinxes to rescue their teacher, Mr. Momoya, who sacrifices himself. During a subsequent battle against the Black Sphinx, Junpei and two classmates are consumed after failing the puzzles.

His documented development occurs within the series' first season.