The Guardian of Forever needs to make its return to Star Trek: Discovery in order to make crossovers happen with the 32nd-century-set Star Trek series. The upcoming Star Trek: Discovery season 5 promises a change in tone from the dire, galactic threats of seasons 3 and 4. Captain Michael Burnham (Sonequa Martin-Green) and the USS Discovery are chasing "the greatest treasure in the known galaxy." Whatever this prize is in Discovery season 5 sets up more galactic exploration than in prior seasons – which could lead to the return of the Guardian of Forever.
Introduced in what is considered the greatest Star Trek: The Original Series episode of all, "The City on the Edge of Forever," the Guardian of Forever was a sentient time portal discovered by Captain James T. Kirk's (William Shatner) Starship Enterprise. The oblong-shaped Guardian existed on a long-dead planet, and it was the vessel through which Kirk and Spock (Leonard Nimoy) time-traveled to 1930s New York on a hunt for a rogue Dr. Leonard McCoy (DeForest Kelley) and save the timeline. The Guardian's one-off appearance was unforgettable but over 50 years later, it unexpectedly returned in Star Trek: Discovery season 3.
