A thirteen year old girl named Tabitha Smith, nicknamed "Boom Boom," had not gotten along with her parents for most of her life. Tabitha's superhuman power to create balls of explosive concussive energy, which she calls her "time bombs," emerged when she was thirteen, and when her parents discovered them, they were appalled. Resentful towards her father, Boom Boom put a small explosive ball of energy in his lasagna. In retaliation, her father beat her severely. Apparently, Boom Boom's father also beat her on other occasions after he discovered her mutant powers. Somehow learning there was a school for mutants in New York's Westchester County (which was, in fact, Professor Xavier's School for Gifted Youngsters, headquarters for the X-Men and the New Mutants), Boom Boom started journeying there by train. She decided to give herself the alias "Time Bomb."
Near Washington, D.C. the train was wrecked on a whim by the cosmic entity calling itself the Beyonder. Thinking he was a mutant, Boom Boom accompanied him. The Beyonder abandoned her but returned and took her to Xavier's school. There the X-Men and New Mutants attacked the Beyonder, who escaped. Boom Boom, frightened and bewildered by witnessing the attack, left, but was later found by the Beyonder, who brought her to a distant planet that he called "World Complex Headquarters" of the alien Celestials. There, threatening to destroy the universe, the Beyonder fought and seemingly defeated a number of Celestials. (However, that planet was not truly the Celestials' headquarters, and the Celestials allowed the Beyonder to "defeat" them, presumably in order to observe him in action.) Terrified, Boom Boom demanded that the Beyonder transport her back to Earth and leave her, and the Beyonder complied. Back on Earth, Boom Boom alerted the Avengers about the Beyonder. Summoning the Beyonder, Boom Boom thereby led him into an ambush by the Avengers and other costumed champions. The Beyonder, who had regarded Boom Boom as his only friend, allowed the Avengers to defeat him, but then left. Boom Boom left during the battle.
She eventually encountered the mutant criminal known as the Vanisher, who made her a member of his gang of thieves, the Fallen Angels, and taught her how to steal. Believing that the Vanisher was treating her badly, Boom Boom alerted X-Factor, whom she believed to be a group of mutant hunters, about him. But when two members of X-Factor arrived, who were actually the Beast and Iceman, Boom Boom changed her mind about turning in the Vanisher and used her "time bomb" power on the Iceman as a prank. The Beast and Iceman pursued her and caught up to her in their costumed identities. She agreed to leave the Vanisher's gang and live at X-Factor headquarters instead.
Later, after Boom Boom set off another of her "time bombs" in the X-Factor laboratory as a prank, the Iceman pursued her through the headquarters. Another member of the Vanisher's gang, Ariel, used her powers to enable Boom Boom to escape X-Factor headquarters. Thereafter, Boom Boom stayed for a time with the Fallen Angels. After an adventure on another planet with the Fallen Angels, Boom Boom returned to X-Factor headquarters only to be captured by the anti-mutant organization called the Right. Boom Boom and other mutants associated with X-Factor regained their freedom, however, and she and her fellow X-Factor trainees went into action as the team called the X-Terminators. Still later, Boom Boom joined the New Mutants.
As a member of the New Mutants, Boom Boom became enamored with Samuel Guthrie. She served as a member of the team until they formed the new mutant team, X-Force. Early on, Boom Boom decided to change not just her costume but also her name. For a short time she called herself Boomer. She recently went though a deeper and more drastic change in looks and personality and changed her code name to "Meltdown." She is no longer afraid to use her time bombs in nearly lethal ways. Currently,
Meltdown is a member of X-Force.