How Carl Sagan inspired Star Trek: Discovery's big Season 3 twist

The spirit of Carl Sagan lives on in Star Trek.

Discovery Season 3 features a callback to the classic Trek episode "The City on the Edge of Forever," which propels the story of Philippa Georgiou (Michelle Yeoh) into strange new territory. There are a lot of twists and turns in "Terra Firma, Part 2," and the ending of the episode has major ramifications for the history of Trek and the future of the franchise, too.

Ahead of the debut of "Terra Firma Part 2," Inverse spoke with the writers of the episode — Bo Yeon Kim, Erika Lippoldt, and Kalinda Vazquez — to discuss the nitty-gritty of Trek time travel, what's next for Georgiou, and why they brought back a very famous Star Trek plot device all the way from 1967.

Major spoilers ahead for Star Trek: Discovery through Season 3, Episode 10.

After spending roughly two episodes back in the Mirror Universe, Philippa Georgiou returns to the surface of the planet Dannus V, only to be faced with yet another choice. She can't go back to the Mirror Universe, and she can't stay in the Prime Universe either. But, with the help of the mysterious Carl (Paul Guilfoyle), there's another path.

But who is Carl? Turns out, he's actually the personification of the Guardian of Forever, a time portal first seen in perhaps the most famous — and most beloved — classic Harlan Ellison-penned Star Trek episode, "The City on the Edge of Forever." Hiding on a distant planet — different from the one Spock and Kirk visited — the Guardian offers its services to help take Georgiou to a new time and place.

The Discovery writers tell Inverse they wanted the resolution to Georgiou's time-sickness to have a timey-wimey solution. This means they considered some other options, including The Next Generation's enigmatic Q, or a "time weapon that was confiscated during the Temporal Cold War."

Ultimately, they decided the best option was the Guardian of Forever.

"We’ve been itching to find a way to use the Guardian on Discovery," Kim and Lippoldt tell Inverse. "The Guardian also provides flexibility in that it has only appeared in that episode, and The Animated Series episode "Yesteryear" — which somewhat amusingly broke the rules established in the TOS episode — so we felt there was room to do something new and different with it."

The writers also reveal they almost went a totally different direction with the episode. The original plan didn't involve the Mirror Universe at all.

"Burnham and Georgiou were going to beam down to the planet expecting to find the Guardian, and instead come across a lone house with a garden — not unlike the house in TNG’s “The Survivors." Its sole occupant, a mysterious Gardener, would turn out to be the Guardian," the writers explain. "Georgiou was going to live out her final hours there choosing to have a perfect day with (Prime) Burnham -- it involved baking an apple pie!"

The team says we didn't see this pie-baking scenario because they "realized that a much larger, cathartic story" was needed to make the final journey of Georgiou pack a larger punch.

Speaking of apple pie, the late-great Carl Sagan once said: "If you wish to make an apple pie from scratch, you must first invent the universe." In Discovery, the writers may not have gotten their apple pie inside of a pocket-universe, but they did name the new persona for the Guardian of Forever directly after the famous astrophysicist.

"When we discussed the personification of the Guardian of Forever, we wanted to create a being who seemed both omniscient, but also perpetually curious about all things in the universe," Vazquez explains. "Carl Sagan was brought up by our showrunners Michelle Paradise and Alex Kurtzman as a potential analog because of his tendency to philosophize about all the unknown the galaxy holds. Sagan was definitely an inspiration for Carl!"

Fans of Sagan's fiction might note that the functioning of the Trek portal is similar to time travel depicted in the movie version of Contact, based on a novel by Sagan. In both instances, people journey across time and space, but from the perspective of the real world, only minutes have passed. Like Dr. Arroway's hours of static videotape, Georgiou similarly has "three months worth of biodata" in her nifty little wrist monitor.

The departure of Michelle Yeoh's popular character at the end of this episode seems to set up the long-discussed spin-off series, tentatively called Section 31. Producer Alex Kurtzman has said the setting of this show will be "surprising," and now we have a clue as to why. We don't know exactly where in the timeline the Guardian is sending Georgiou, but he says that it will be at a time "when the Prime Universe and Mirror Universe were still aligned." So, does that mean Georgiou will end up in ancient Rome? The early 20th Century? How about a Star Trek spy show set in 2021?

"Georgiou is sent back to a point in time that she doesn’t expect… But it ends up being exactly where she needs to be," the Discovery writers tell Inverse. "A lot of care was put into developing Georgiou’s arc this season. As for what it sets Georgiou up for beyond that — only space-time will tell."

Star Trek: Discovery is streaming now on CBS All Access.