Ultraman Joneus hails from Planet U40 within the Nebula M78 universe. He encounters astronaut Choichiro Hikari during a critical space mission, proposing a merger after Hikari's spaceship suffers an accident to combat imminent extraterrestrial dangers threatening Earth. This fusion enables Hikari to transform into the 40-meter-tall Ultraman Joneus as needed while retaining his human identity daily.

Joneus operates with the Science Guard Party, an Earth defense team investigating global mysterious luminous phenomena. His core mission is defending Earth from kaiju and alien invaders. Distinct among Ultras, Joneus lacks a separate human form; his existence remains inextricably bound to Hikari, who maintains full consciousness during transformations. His capabilities include flight, super strength, energy projection, and wielding the Ultra Star Bracelet for specialized attacks.

Throughout his battles, Joneus faces adversaries like the Heller Empire and other cosmic forces. His journey includes explorations of Planet U40, revealing his origins and the wider Ultra civilization. The character's narrative emphasizes duty and sacrifice, particularly through the symbiotic bond with Hikari, who must balance human life with his role as Earth's protector.