An archeological group found an untouched tomb in Egypt. A fight broke out among them (the group was comprised of a married couple - CC and Lynn Batson - and Theo Adam) over how things should be handled. While Lynn ran for help, the two men fought. Adam stabbed CC in the back as the tomb began to crumble. He grabbed what he could and ran. Outside, he killed another man and headed out to find Lynn Batson.
He caught up with her in her hotel and killed her. He also killed the caretaker of their children. Adam grabbed their daughter, Mary, saying her parents were away and he would take care of her until they returned.
A few months later, back in Fawcett City, a young boy named Billy was taken into caverns beneath the subway by a mysterious man. There, he met a wizard who introduced himself as Shazam. By saying the old man's name, Billy was transformed into Captain Marvel. Confused by his new body and strength, Billy flipped out. Shazam told him he must fight for the poor and against evil. The strange man returned. Captain Marvel spoke the wizard's name once more and returned to his normal self.
TB Sivana, the man who'd funded the Egyptian expedition, was visited by authorities. They told him they believed the Batsons were murdered by their colleague, Theo Adam. He claimed to be a victim in the situation as well, and ordered the man (and the man who was attacked at the site) out of his office. Sivana went to Adam and ordered him to silence the men.
Young Billy Batson witnessed Adam and his men setting explosives at a radio tower. He confronted them, but they tossed him from the roof. On his way down, he summoned the wizard and became Captain Marvel. Adam confused him for CC Batson and fled. Captain Marvel threw the explosives toward the harbor, but overshot. They landed on the site where the World's Fair would be held and detonated. He took down all the bombers except for Adam.
After a near mistake (saying "shazam" while flying), Captain Marvel headed back to the subway to question the wizard. He bumped into the school janitor, Mr. Dudley. He also saw a vision of the wizard.
Sivana went to Theo Adam. He began putting the pieces together - the spitting image of CC Batson and the lightning bolt on Captain Marvel's outfit (which matched something he'd seen in the tomb). He grabbed the jewels he'd stolen and yelled Shazam. Theo Adam was transformed into Black Adam. He set out to find the Batson's son.
He found Billy's stuff (Billy had been living on the streets) and was approached by a cop. He tossed the cop into an oncoming car, which just happened to have his missed kill from Egypt and the Egyptian official inside. A thunder clap was heard and he turned around to see Captain Marvel. As they fought, he let loose that he'd killed the boy's parents. After a huge fight Captain Marvel realized Black Adam's power came from the necklace he'd stolen in Egypt. He snatched it and his adversary turned into the man who'd tossed Billy Batson from the rooftop.
The wizard removed his memories of Black Adam; Captain Marvel handed him and the necklace over to the police. He called out to the wizard. They talked about the power; Billy's father (the mysterious man who led him into the subway) convinced him to keep it. They told Marvel that his sister was still alive.
Billy posed as his uncle Eben to fool his teacher.