Tempus first appeared in the episode "Tempus Fugitive" (2.18). He was brought to Metropolis by H.G. Wells, who had discovered him on his travels to the future and was looking for a volunteer to help him prove to his associates that he had indeed traveled to the future. Tempus detested this future, a Utopia created by Superman's descendents and founded on the values and principles by which Superman led his life. According to Tempus, Utopia was "[a] world of peace. A world with no greed or crime. A world so boring you'd blow your brains out but there are no guns.... No one works. No one argues. There are nine thousand channels and nothing on!"
In order to destroy Utopia, Tempus, who, being from the future, was aware of Superman and Clark Kent being one and the same, decided to travel back to Smallville in 1966 and kill Clark as a baby. With the help of H.G. Wells and a detour to 1866 Smallville, (where he meets and embraces Jesse James and his brother Frank) Lois and Clark prevented this from happening, but during the course of events, Tempus revealed Clark's secret to Lois, uttering the memorable line, "Hello! Duh! Clark Kent is Superman!" By the end of the episode, however, Lois and Clark's memories were erased and the status quo was reestablished.
While Tempus did not directly appear in the season two episode "And the Answer Is...," he did play an important role. While imprisoned in the Wild West after the events in "Tempus Fugitive," Tempus wrote a diary in which he revealed Clark Kent's secret identity. The diary had fallen into the hands of Jason Mazik, who attempted to use this information to blackmail Clark into stealing for him. When Clark did not cooperate, Mazik kidnapped Martha and Jonathan Kent. With the help of Lois Lane, who is willing to sacrifice her own life to save Clark's parents, Mazik was stopped and the diary destroyed.
Tempus reappeared in the season three episode "Tempus, Anyone?" where he kidnapped Lois Lane and brought her to an alternate timeline. In that dimension, Clark Kent had not created Superman and was engaged to Lana Lang, while Lois Lane had been lost on assignment in the Congo. Lois helped the alternate Clark become Superman, only to have Tempus expose his secret identity to the world on television. At the end of the episode, Wells returned Lois to her own universe and Tempus was arrested.
In the season four episode "Soul Mates," Lois's and Clark's souls traveled back in time (assisted by H.G. Wells), inhabiting previous incarnations of themselves, and Tempus appeared again. His earliest incarnation was Baron Tempos, the nemesis of Sir Charles (aka The Fox) and Lady Loisette. He also appeared as Tempus Tex, archenemy of The Lone Rider and Lulu.
Intent on destroying Lois and Clark's relationship, Baron Tempos had placed a curse on Lady Loisette and Sir Charles, which prevented them from consummating their marriage, lest a horrible tragedy befall them and all incarnations of their souls. In the end, Lois and Clark were able to lift the curse and return to their own time and the present incarnations of their souls.
In the season four two-parter "Meet John Doe"/"Lois & Clarks," after escaping Andrus (played by William Christopher of M*A*S*H fame) as he was being transported to a prison in Utopia, Tempus returned to Metropolis as "John Doe," a candidate opposing the current President of the United States in the presidential election. With the help of advanced technology from the future, Tempus was able to brainwash the people via the telephone into thinking he (John Doe) was "a darn nice guy," and won the election by a landslide. Tempus only complaint was not it was not a complete sweep of all the citizens. When he asked why the Amish didn't vote for him his aid said that they don't have telephones.
At the end of "Meet John Doe," Tempus employed a trick to banish Superman to a time vortex, a place outside of time. In part two, "Lois & Clarks," H.G. Wells brought the alternate Clark (from "Tempus Anyone?") to Metropolis to help stop Tempus, and to keep up the illusion that only Clark Kent, and not Superman, was missing. Lois and Alt-Clark were able to thwart Tempus's plans and rescue Superman from the time vortex, and Tempus was arrested. This was his last appearance on the show.
To this day, Tempus is "so wildly popular, fans voted him a favorite villain over Lex Luthor.
Lois and Clarks
Tempus: Well I'll be damned! He did it! Does he think that's all it takes to finish me? All that spandex must keep the blood from his brain!