At the Rock of Eternity, Shazam tells Billy that he has a baby sister. Then Billy finds himself sleeping at his place. On the television a news report is on. The authorities still have no idea where the giant crop circles in the shape of footprints or the alligator creatures came from. Billy notices that the treads on the footprints match his shoes. He wishes he had listened to Captain Marvel's warning about climbing the Rock of Eternity. Dr. Sivana, head of the Department of Technology and Heartland Security promises he will get to the bottom of the monster phenomenon. Billy sees an ad for the circus that is in town with its Monster Society of Evil act. He decides to go check it out.
He catches a news conference with Dr. Sivana telling the people that they will get to the bottom of this mystery. Ducking out, he sees a swarm of ants all going in the same direction. Then he stumbles into Lagreen and a friend of his. Lagreen is still demanding that Billy pay him to live on "his" street. Running away, he makes his way and sneaks into the circus. Lagreen follows just as the Monster Society of Evil act begins.
The act is going wrong as the alligators eat the announcer. They begin to speak to the crowd. They do not want to eat everyone...just the children. As they grab a child, Billy notices that it is his sister that he saw in the vision at the Rock of Eternity. He says "Shazam!" and is transformed into Captain Marvel.
As Captain Marvel fights the alligators, a talking tiger walks up. He reveals that he is not on the side of the alligators. The alligators ask him what will their leader, Mr. Mind, say. The tiger responds that he doesn't work for Mr. Mind.
With all the alligators defeated and locked up, Captain Marvel notices that the tiger has disappeared along with Billy's sister. Rushing outside, he sees an enormous creature that calls out to him. It says it is Mr. Mind and is there because of Captain Marvel. It came from beyond. He wants to remove all traces of civilization from this world. Captain Marvel tries to defeat Mr. Mind but is unable.
Flying off to find Billy's sister, he finds her by the water front with the tiger. It turns out the tiger is Talky Tawny, who Billy used to run errands for. Talky also works for Shazam and has been helping Billy adjust to his new life. Mary asks to live with him. She reveals that she doesn't have any parents either and ran away from her foster mother.
Two creatures now can be seen. Billy finally agrees that it is up to him to clean up his own mess. Transforming again, a stray bolt of lightning hits Mary and sends her smashing into a parked car. Rushing to the car, they find little Mary okay and dressed in a costume similar to Captain Marvel's. She is also able to fly just like him. Captain Marvel now has to keep her under control and defeat the two creatures.