When Willie rubs the magical buckle on his belt and speaks the word "Thunder", he is transformed by a brilliant starburst and a loud thunderclap into the adult Captain Thunder. The Captain possesses super-strength, super-speed, invulnerability to physical harm and the ability to fly, the levels of which make him a peer to Superman in sheer might. In addition, the belt buckle's magical energies give Captain Thunder great wisdom, bravery and tenacity. By rubbing the belt buckle, speaking his magic word and willing it to do so, Captain Thunder can use its power to break the time barrier and travel between dimensions.
One night, while on a Summer camping trip with a group of fellow orphans, Willie Fawcett noticed a mysterious owl flying overhead, which seemed to beckon the youth to follow it. Pursuing the owl towards the solid rock wall of a hillside, the rock face suddenly opened into a cavern. Willie followed the owl inside and, to his astonishment, encountered Merokee, the last of the Mohegan tribe's great medicine men. Merokee explained that a tribal legend told of a boy of noble spirit who would come forth and be granted great powers by the last of the Mohegan shamans, and that Willie was the boy described in the legend. Holding up a belt with a lightining bolt emblazoned on its buckle, Merokee told Willie that, upon rubbing the buckle and speaking the word "Thunder", the boy would be invested with seven magical powers as follows: Tornado (power), Hare (speed), Uncas (bravery), Nature (wisdom), Diamond (toughness), Eagle (flight) and Ram (tenacity). Willie rubbed the buckle and spoke the magic word as instructed, and was instantly transformed by a brilliant starburst and a clap of thunder into the powerful hero called Captain Thunder. Proclaiming to the Great Spirit that his work was done and that he was ready to pass on, Merokee was transformed into a spirit and soared skyward, leaving Captain Thunder to do battle with the forces of evil whenever and wherever they arose.
Throughout the 1950s on the Earth of his home universe, Captain Thunder battled all manner of crimes and injustices across the globe, becoming that world's foremost hero. His greatest battles were against a coalition of incredible creatures known as the Monster League of Evil, whom he fought across 1,953 different dimensions and finally imprisoned in a misty purgatory-like realm. As Captain Thunder was leaving to return to his native dimension, the Monster Leaguers placed a curse on the Captain that would cause him to turn evil when next he assumed his heroic guise. Through some unknown mishap, Captain Thunder (in his Willie Fawcett identity) wound up in and alley of Metropolis on mainstream Earth in 1974. Given the fact that Willie believed Metropolis to be his home city on his native world, it can be inferred that Captain Thunder had his base of operations in a parallel version of Metropolis (albeit in the past). When Willie witnessed an armored car being robbed, he transformed himself into Captain Thunder with the intent of capturing the criminals, but turned to evil as the Monster League had planned. Instead of apprehending the crooks as Willie indended, the Captain began aiding them in the robbery, which brought him into conflict with Superman. The two powerful heroes engaged in a brief skirmish, but Captain Thunder managed to elude the Man of Steel by changing back to Willie Fawcett, who had no memory of what had taken place.
Willie went to the Galaxy Communications Building to find Superman (whom he had seen just prior to transforming into Captain Thunder) in the hope that the Action Ace could help him out of his weird predicament. Willie was directed to Clark Kent (Superman's alter-ego), to whom Willie told of the circumstances that lead to him appearing in Metropolis. When Willie, Clark and Lois Lane went to Metropolis Police Headquarters to ask if a Willie Fawcett had been reported missing, the three witnessed the same crooks who had been involved in the armored car heist trying to break their leader out of jail using a strange aircraft. As Clark slipped away to switch to Superman, Willie also ducked out of sight to change to Captain Thunder, who was again seized by the compulsion to perform acts of evil. Instead of aiding Superman in stopping the criminals, the Captain attacked the Man of Tomorrow and the two became embroiled in a titanic struggle that was carried over into the mountains outside the city. At the battle's conclusion, Superman tricked Captain Thunder into changing back to his mortal form, once again regaining his senses in the process. Realizing that he was in the wrong dimension, Willie transformed himself back into Captain Thunder and (while Superman held the Captain in a full-nelson) forced himself to use his wisdom to break the curse that the Monster League placed on him. Bidding farewell to Superman, Captain Thunder then used the power of his magic word to return to his native universe (S No. 276, June 1974: "Make Way for Captain Thunder!").