The son of Professor Morton Kirk, a noted astronomer who, approximately a decade ago, while Johnny was still an infant, spotted â€œa giant planet, plunging from outer space" on a collision course with Earth. Believing that a â€œcosmic collisionâ€ is inevitable and that â€˜â€˜the end of the worldâ€™â€™ is at hand, Professor Kirk places young Johnny inside â€œa space rocket he had recently perfected" and fires him off into outer space toward a hoped-for haven on a distant planet, much as Jor-El and Lara once launched the infant Superman into the interstellar void to save him from the death of Krypton.
However, the Earth is rescued from the impending cataclysm through the heroic intervention of the then teen-aged Superman, but Professor Kirk, profoundly shaken by the shock of having needlessly exiled his son into space, dies soon afterward, leaving behind him a document appointing Superman as his sonâ€™s legal guardian in the unlikely event the youngster ever succeeds in returning alive from space.
In September 1957, however, Johnny Kirkâ€”now approximately twelve to fourteen years of ageâ€”does return, having survived since infancy on a far-distant "primeval planet,â€ at least in part because he had become endowed with super-powers identical to Supermanâ€™s when the rocket built by his father passed through â€œa peculiar cloud in space, pulsating with energy," a mysterious "cloud of cosmic energy," that somehow â€œincreased his capabilities far beyond normalâ€¦!â€
Determined to comply properly with Professor Kirkâ€™s last request, Superman thows himself enthusiastically into the role of foster parent, proudly dubbing his young charge Superman Junior, and launching him on a rigorous training program designed to prepare him for the demanding role of his own junior partner in the father and son superhero team of Superman and Superman Junior.
Complications arise, however, when Supermanâ€™s powers begin one by one to fade and vanish as the result of his having breathed in the noxious fumes of the â€˜â€˜peculiar meteoric dustâ€™â€™ clinging to the wreckage of Johnny Kirkâ€™s rocket, an unearthly variety of â€œsuper-poisonâ€ that would in time render the Man of Steel as vulnerable as an ordinary earthman.
However, in spite of Superman's courageous effort to keep his dilemma a secret, Johnny becomes aware of Supermanâ€™s increasing disability and willingly sacrifices his own hopes for a superhero career by transferring his own super-powers into the Man of Steel's body with the aid of a special meteor whose unique properties enable him to effect a permanent â€˜â€˜transfusion of power" from his own body to Supermanâ€™s, albeit only at the expense of transforming the boy into an ordinary earth boy (Act No. 232: "The Story of Superman, Juniorâ€).