Dr. Don Blake treats Mike, the Circus of Crime strongman who performs as Thor, who has wrenched his back lifting something the Ringmaster won't divulge. Blake remembers Thor's earlier encounters with the Circus of Crime and, thinking they're involved with a theft at the electronic center, decides to investigate as Thor.
In Asgard, Loki gathers various malcontents to plot against Thor, including Ulik who demands to be sent to fight the Thunder God. Loki complies, sending Ulik to the Ringmaster's circus. There, Ulik uses his great strength in destruction until the Ringmaster hypnotizes him. He hopes that Ulik will have the needed strength to finish Mike's task.
Meanwhile, Thor pretends to be Mike and performs for the circus. Ringmaster unveils Ulik, instructing him to load a 5,000-pound barbell on a truck. Realizing the Ringmaster's plan, Thor awakens Ulik who attacks him. Thor uses Mjolnir to transport Ulik back to Asgard, helps the police round up the Circus of Crime, then reveals that the barbell contains the government's secret dual computer stolen from the electronic center.