Born to a scientist father and an extraterrestrial mother, Jack Hart unknowingly developed energy abilities that threatened to consume him. His father, concealing the truth from him, developed a radioactive inhibitor called Zero Fluid, which he claimed was a new form of fuel. When Philip was slain by agents of the criminal Corporation, Jack was accidentally doused with Zero Fluid, which served as a catalyst to activate his energy powers under limited control. Developing armor to contain his energies and calling himself the Jack of Hearts (in honor of his father's love for playing cards) Jack set out to avenge his father's death. This initially brought him into conflict with several heroes including the White Tiger. After slaying his father’s murderer, Jack was taken under the wing of Iron Man and became active as a hero. When he began losing control of his power level, he turned himself in to S.H.I.E.L.D., where he was isolated and trained.
Jack subsequently learned of his alien heritage and traveled to his mother's homeworld, Contraxia, where he re-energized the planet's dying sun. Unwilling to return to Earth, Jack remained in space for a while and was briefly captured by the Stranger and held prisoner on his Laboratory World. The Laboratory World prisoners were set free during an invasion by the Overmind and the Squadron Supreme, and Jack went back to Earth. Upon his return to Earth, Jack battled Quasar and fellow escapees the Presence and Starlight, and the Presence blew a hole in Jack's suit, causing him to retreat into space. Moondragon retrieved him from space, revived him, and mentally forced him to battle Kismet. After he escaped Moondragon's control, Jack set out for space again.
Jack sought out the Silver Surfer‘s aid in controlling his powers and Jack received an improved set of alien armor on the Anvil prison planet where he helped the Surfer battle Nebula and Gaetar. A short time later, he was captured by the powerful Tyrant along with Thor‘s oath-brother Beta Ray Bill, the leader of the Imperial Guard, Gladiator, alien warrior woman Ganymede, and former Heralds of Galactus, the Silver Surfer, Terrax, and Morg (the latter of which was Galactus' Herald at the time). All except Morg managed to escape from Tyrant, and Jack became a close ally and lover of Ganymede.
Later, Jack encountered a group of Avengers in space and was involved in a conflict between the Avengers and the Infinites. Jack accompanied them back to Earth, coming into conflict with the alien Ruul (who were secretly just a mutated faction of the Kree) when they plotted to turn the Earth into a prison planet for inter-galactic criminals. After this conflict, he accepted the invitation to join the Avengers. Unfortunately, remaining on Earth required him to subject himself again to periodic isolation, this time for fourteen hours a day. Growing tired of such restraints, Jack allowed himself to explode due to a buildup of his energy, but not before saving Ant-Man‘s daughter Cassandra Lang from a child-killer. He was believed to be dead for a time but apparently returned when the Scarlet Witch went insane (though his zombie-like form might have been an apparition created by the Witch) and his body exploded again, killing Scott Lang in front of the Avengers Mansion.