10 Star Trek Episodes That Paid Off WAY Later
2. The City On The Edge Of Forever
The Guardian of Forever was introduced at the end of the Original Series' first season in the episode The City On The Edge Of Forever. It would return in the Animated Series episode Yesteryear. In both cases, it is a stable method of temporal travel, though it clearly has a mind of its own.
Altering the timeline is referred to as one of the greatest headaches for Starfleet, or simply as 'Tuesday' for James T. Kirk. However, this entry is less about Kirk as, ironically, it is he who ensures the stability of the timeline after his encounter with the device. Rather, it is Starfleet themselves, and others who engaged in the Temporal Cold War, who would come to abuse the artifact.
Star Trek: Discovery's third season two-parter, Terra Firma, sees the return of the Guardian to the franchise. This time, it appears initially in a different form. Carl, as it likes to be called, is presented as a man who speaks in riddles, holding a copy of the Star Dispatch, prophecizing Georgiou's death. This leads to the jaw-dropping reveal of the Guardian, realized in 21st-century graphics, rendered beautifully for the modern day.