Ouji Tanaka, originally known as Gabriel during his music career, served as lead guitarist for the short-lived heavy metal band Black Heaven roughly fifteen years before the story's main events. The band briefly achieved fame as a one-hit wonder. His trajectory shifted dramatically when a groupie, Yoshiko, became pregnant with his child. Choosing responsibility, Ouji abandoned his musical ambitions, married Yoshiko, and adopted the conventional life of a salaryman in a mundane office job to support his family. This transition eroded his former identity, accelerated by Yoshiko systematically discarding his band memorabilia – records and equipment – over the years due to their small apartment's space constraints.

Now middle-aged and a section manager, Ouji exhibits signs of midlife dissatisfaction, chafing under routine and feeling unfulfilled by his corporate role, clinging only to fading memories of his rockstar past. His reluctance to revisit music stems from familial duty and Yoshiko's disapproval; she views his guitar playing as a disruptive relic. Domestic tension escalates as Yoshiko suspects Ouji of having an affair when he starts disappearing for extended periods.

Ouji's life transforms upon encountering Layla Yuki, an alien military officer disguised as his coworker. She recruits him to play his Gibson Flying V guitar, revealing his music unknowingly powers an extraterrestrial ultimate weapon critical to an interstellar war. Despite initial disbelief and prioritizing commitments like reluctantly babysitting his son Gen, Ouji gradually reengages with his passion. His performances generate destructive energy waves against enemy forces, though he remains largely unaware of the direct combat impact initially.

This involvement with Layla's mission reignites Ouji's artistic identity, prompting him to reunite with former bandmates Satou (Michael), Yamada (Luke), and Suzuki (Raphael). Their reunion is complicated by adult responsibilities, requiring negotiations around jobs and families. The band's resurgence faces direct threat when their deceased keyboardist, Watanabe (Joseph), is resurrected as an eight-armed cyborg, "Mechanabe," by enemy forces. A climactic battle ensues where Ouji's emotional depth and musical expression, channeled through a power ballad, overwhelm Mechanabe's mechanical precision, securing victory.

Throughout this journey, Ouji navigates persistent friction with Yoshiko, who interprets his absences and secrecy as infidelity. Their relationship evolves from conflict toward mutual understanding, particularly after Yoshiko gifts him a new Flying V guitar, symbolizing reluctant acceptance of his dual life. Concurrently, his bond with son Gen strengthens; Gen idolizes him after witnessing his heroics during an alien ship incident, associating him with characters from his favorite space-themed show. By the resolution, Ouji achieves equilibrium, integrating his musical passion with familial duties without fully reverting to his "Gabriel" persona. He continues playing guitar with a renewed focus on emotional expression rather than reliving past glory.