David Tennant as the fourteenth doctor in the Doctor Who 60th anniversary special
Science fiction is one of the most daring genres of television, thanks in part to its ability to go to places other genres wouldn't dare to go. By taking risks, these series have the potential to be historic as they reshape storytelling and bring audiences to new frontiers.
Of course, not every risk comes with a reward, but when it does, oh, it's glorious. In every great science fiction series, there's one major episode that changes everything. Whether opening up new possibilities of tone, changing the course of the story, or revealing a major twist that sets up the future, these episodes are game changers. From major season finales to pivotal mid-season thrills, we're here to celebrate these monumental episodes that also happen to be some of the greatest of all time. 1 'Black Mirror' Season 3, Episode 4 "San Junipero" When you think of Black Mirror, you immediately think of how the dark, dystopian thriller paints a bleak picture of our distant technological future. But then came a Season 3 episode and the entire potential of the series changed forever. In"San Junipero," written by series creator Charlie Brooker, a melancholic look into a simulated reality is explored with an uplifting tone of companionship and nostalgia, reshaping the types of stories the thriller series could tell. The critically acclaimed episode tells the story of a 1980s-set virtual reality in which elderly people and the deceased can inhabit young, simulated bodies."San Junipero" follows the romance between the shy Yorkie and the outgoing Kelly through the technological afterlife, where they find love in a simulated paradise. The episode was groundbreaking in an assortment of ways that made it memorable. Between the beautiful depiction of LGBTQ+ characters and indicating a more positive prediction of how future technology can be used for good,"San Junipero" pushed the boundaries of where Black Mirror could head. The brightly colored visuals, the emotionally tinged performances, and the uplifting message were so well received that future seasons tapped into more nuanced tones in their stories."San Junipero" is an emotionally refreshing episode that revitalized the already beloved program. 2 'The Twilight Zone' Season 1, Episode 22 "The Monsters Are Due on Maple Street" The brilliance of The Twilight Zone is the anthology series’ ability to tap into our greatest fears through disturbing and unusual events. Presented by Rod Serling, the standalone episodes kept audiences on their toes, especially when they would hit closer to home. Such was the case with the Season 1 thriller,"The Monsters Are Due on Maple Street." Written by Sterling, the episode tells the story of a meteor that causes a neighborhood blackout, leading residents to succumb to paranoia and fear, turning on each other with accusations of being disguised aliens, ultimately revealing the true monsters are their own prejudices and capacity for mob mentality, manipulated by unseen extraterrestrials. A major reflection on society through the allegory of McCarthyism and the fragility of society when fear takes hold,"The Monsters Are Due on Maple Street" was a cautionary tale that still resonates today. The practical approach to the episode made it an utterly terrifying story. Yes, there was a science fiction concept to the story, but by keying in on humanity as the central monsters proved, The Twilight Zone is scariest when it's holding a mirror up to society. It's no surprise that it's one of the greatest episodes of the original series. It's so incredible, the 2019 reboot remade the episode with a newfound theme of terrorism. It's clear that Sterling's story is terrifyingly poignant and timeless. To be frank, another remake could be made today, and it would still resonate. 3 'Severance' Season 1, Episode 9 "The We We Are" Perhaps one of the biggest new series of the last five years, Severance has served as a game-changing show. Only two seasons in, the Dan Erickson-created series has reshaped science fiction stories on television forever. But there's one Season 1 episode that changed the entire series:"The We We Are." In the Season 1 finale, Dylan's activation of the"overtime contingency," the Innies wake up in the outside world for the first time. They experience their Outie lives, discovering shocking truths about their identities, relationships, and the sinister nature of Lumon Industries. A high-tension episode, the finale's brilliant evolution of the cinematic world pushed the story to the next level, highlighting the ethical, personal, and emotional cost of the"severance" procedure. The most high-octane thrill ride the series can provide, the highly satisfactory episode sprinkled in subtle answers while simultaneously opening up further wonderment. The breakthrough the Innies made changed the entire direction Season 2 would head. Each character's journey was utterly fascinating to watch as they endured different journeys. Mark wakes up in his sister's home to his Outie's grief and the truth about Ms. Cobel . Irving finds his Outie's obsession with Lumon, painting the same dark hallway and investigating fellow employees. And then, of course, Helly discovers she is Helena Eagan, daughter of the Lumon CEO, and that her Outie forced her into this role to prove the procedure's safety. Every single individual story changes the future of the story in such a brilliant fashion. 4 'The Last of Us' Season 1, Episode 3 "Long, Long Time" There has been an extraordinary trend of bringing iconic video games to life on screen. Perhaps the biggest series of such is The Last of Us. Set decades after the collapse of society caused by a mass fungal infection that transforms its hosts into zombie-like creatures. The series focuses on Joel's mission to escort immune teenager Ellie across post-apocalyptic America. In the first two episodes, it was all Ellie and Joel. By Episode 3, the plot shifted dramatically, leading to one of the most emotional episodes of television ever. In the episode, Joel and Ellie set out to Lincoln, Massachusetts, to meet Bill . The story flashes back 20 years as he survives in his town and meets his partner, Frank . The beautiful episode expanded Bill’s backstory while exploring a deeper look at love, happiness, and the passage of time. Not only did the series take a pause from non-stop action and horror, but it also proved that emotional connections are integral for the betterment of a television series of this type. Wonderfully rich in performance, Offerman and Bartlett dominated, earning both critical praise and award nominations. With humor and heart, it exposed sentimentality, putting the rest of The Last of Us' journey in a new perspective. Further, by briefly straying from the source material, it afforded new opportunities as the series progressed. From an LGBTQ+ storytelling perspective, it's one of the single most incredible standalone episodes in modern television. 5 'Lost' Season 3, Episodes 22 and 23 "Through the Looking Glass" We all may still be a bit miffed at how Lost concluded, but surely we cannot be mad at some of the exceptional moments on the journey to the end. One of the biggest water cooler series of the early aughts, the hit ABC series was appointment television. You needed to be caught up on the twists and turns because they were so mind-blowing. When Lost was around, it was the time to have nearly two dozen episodes per season, unlike the eight to ten concise episode seasons. By the time Season 3 arrived, the two-part season finale may have given fans fatigue as they were patiently awaiting answers to literally anything. What we were given in"Through the Looking Glass" wasn't just clues; we were given a new plot device that changed the entire trajectory of the series. The episode's basic premise followed Jack and the survivors as they waited for Charlie to turn off The Others' signal-jamming device, unaware that he had been captured at the Looking Glass station. The most iconic image and moment from the episode will forever be Charlie’s hand with the words written"Not Penny's boat." But the most crucial element of the episode was the flash-forward, in which we meet Jack and Kate discussing the island at a funeral parlor named Hoffs/Drawlar. Why is that name a significant key? It's an anagram for"flash forward." The mechanic was a game-changing plot device because it not only gave us insight into the ultimate survival of key characters, but it began a narrative that shifted from the fight to survive to the consequences of their actions and the desperate, often destructive, motivation to return. 6 'Buffy the Vampire Slayer' Season 4, Episode 10 "Hush" Buffy the Vampire Slayer was a cult classic series in an assortment of ways, primarily placing a female action heroine into the forefront of her own series. Sarah Michelle Gellar became iconic for her portrayal of Buffy, from her teenage years to adulthood. There were certainly many great episodes, but not one that changed the game quite like the Season 4 episode"Hush." Written by series creator Joss Whedon, the story is essentially a dark fairy tale involving a group of demons called the Gentlemen. The terrifying suited demons float into Sunnydale, where they steal everyone's voices, ensuring that the next night, when they start cutting out people's hearts, they can't scream for help. An utterly terrifying concept, the episode proved that Buffy was more than its witty dialogue. It was enriched by silence. Through non-verbal storytelling to explore communication,"Hush" was a legendary artistic achievement. Through the lack of voice, it showcased how the breakdown of society could be imminent. It also presented individuals being forced to examine their relationships when words literally fail. This mesmerizing episode of Buffy the Vampire Slayer proved that television can be highly experimental while still remaining true to its core. 7 'Doctor Who' Season 3, Episode 10 "Blink" There is certainly an array of game-changing episodes in the new Doctor Who, especially during each regeneration. If we knew what was happening next, we could easily add the most recent episode, where Ncuti Gatwa's Fifteenth Doctor regenerated into an unknown character played by Billie Piper, the actress formerly known as Rose. But the moment the entire series changed was in Season 3 when the most terrifying villains in the new Who era were introduced: the Weeping Angels. In the renowned horror thriller episode, Sally Sparrow must stop the Weeping Angels—statues that move only when not observed—from stealing the TARDIS. The Tenth Doctor is trapped in 1969 and sends cryptic messages through time via DVDs to guide her. Written by Stephen Moffat, the episode not only gave us iconic villains, but it also proved the show could be done with the limited screen time of the Doctor. Considered the scariest episode in series history,"Blink" broke the mold in exceptional ways. By being Doctor-lite, it gave further freedom in future storytelling by giving secondary characters the opportunity to shine in standalone stories. Mulligan was simply phenomenal in her performance, truly proving how terrifying these now-iconic characters would be. Since their initial appearance, the Weeping Angels have played a key role in the show, including in the ultimate demise of former companions Amy Pond and Rory Williams . 8 'Westworld' Season 1, Episode 10 "The Bicameral Mind" Created by Jonathan Nolan and Lisa Joy, Westworld took Michael Crichton's original film and made it even better. The sci-fi Western thriller's first season was filled with wonderment and confusion. The premise was simple, but the desire to learn more about the mysteries of this universe was high. In the Season 1 finale, audiences finally earned the opportunity to learn extraordinarily important plot points that set minds boggling, if you will. At the heart of the episode, it explores the birth of artificial consciousness, where hosts transition from obeying internal"voices" to achieving true self-awareness and free will. It's then revealed that Dolores' past actions were driven by this"two-chambered" mind—a concept from psychologist Julian Jaynes, in which the brain mistakes its own voice for a god's instructions—ultimately leading her to break her loops and embrace her own agency. By discovering that the voice inside is actually her own and not Arnold's , she's officially transitioned from"noble automaton" into a sentient being. Subscribe for deep dives on game-changing TV episodes Join our newsletter to get curated analysis and context on the game-changing TV episodes that reshape sci-fi and genre storytelling. Ideal for fans who want smart takes, episode picks, and deeper perspective across TV culture. Subscribe By subscribing, you agree to receive newsletter and marketing emails, and accept Valnet’s Terms of Use and Privacy Policy. You can unsubscribe anytime. It gets better! The maze was defined as not a physical place for humans, but an inner journey for the hosts to reach the center of their own minds, and achieve consciousness. Oh, but it gets even better! Contrary to early assumptions, Dr. Robert Ford has been assisting in the hosts' evolution, forcing them through suffering to achieve the self-reflection needed for true consciousness. The game-changing finale was not only explosive, but it also opened up a whole new world for the series to explore for future seasons. Even in its violent moments, it’s utterly delightful entertainment. Though the series did seem to fall off as the seasons lingered on, the Season 1 finale became a blueprint for how other short-season stories can end. 9 'Stranger Things' Season 4, Episode 7 "The Massacre at Hawkins Lab" Now that Stranger Things has officially concluded, we can examine the series from a broader lens. The series itself has changed the shape of science fiction thrillers in general, but one episode in particular altered the rest of the story:"The Massacre at Hawkins Lab." In the seventh episode of Season 4, Eleven relives repressed memories of the 1979 murders of her fellow test subjects. It's then revealed that Henry Creel is the true perpetrator, manipulating Eleven and murdering the children to punish them and Dr. Brenner . The mid-season finale changed the understanding of the show's lore and the origin of the antagonist, better known as Vecna. To put it bluntly, the episode recontextualizes the entire series, revealing Vecna as the architect of the Upside Down and the force behind previous events. By taking the burden off Eleven and heightening the dangers of Vecna, the rest of the series raises its stakes exponentially. The episode confirms that Henry Creel is Victor Creel's son, and the first child experiment, 001. Eleven now has more strength after learning how to harness her true power by confronting a repressed, positive memory rather than just anger. Now, in context, it pairs perfectly with Will Byers' full potential coming out through seeing his love in danger. With brilliant storytelling and sensational action,"The Massacre at Hawkins Lab" is top-tier Stranger Things. 10 'Star Trek: The Original Series' Season 1, Episode 25 "The Devil in the Dark" Easily one of the most important sci-fi series of all time is Star Trek: The Original Series. The series that set up an entire beloved franchise follows Captain James T. Kirk and the crew of the USS Enterprise on a five-year mission to explore deep space,"boldly go where no man has gone before," and defend the United Federation of Planets. Within the entertainment, the show explored 1960s societal issues, including racism, war, religion, and politics, through a utopian, forward-looking lens. In the Season 1 episode,"The Devil in the Dark," the Enterprise investigates a mining colony on Janus VI, which is being terrorized by a silicon-based creature known as a Horta that is killing miners. Spock discovers the Horta is not inherently evil, but is protecting its eggs, leading to a peaceful resolution based on mutual understanding. Highlighting the fear of what's not understood, the episode reshaped the series by embodying the series' core values of empathy, understanding, and logic over violence when confronting the unknown. The monster story, instead, is one of morality. It also happened to feature Spock's infamous line,"I'm a doctor, not a bricklayer," for the first time. By effectively balancing suspense, moral philosophy, and character development, Star Trek: The Original Series was put into a new perspective forever. Like Follow Followed Star Trek: The Original Series TV-PG Drama Action Adventure Sci-Fi Release Date 1966 - 1969-00-00 Network NBC Showrunner Gene Roddenberry Directors Marc Daniels, Joseph Pevney, Ralph Senensky, Vincent McEveety, Herb Wallerstein, Jud Taylor, Marvin J. Chomsky, David Alexander, Gerd Oswald, Herschel Daugherty, James Goldstone, Robert Butler, Anton Leader, Gene Nelson, Harvey Hart, Herbert Kenwith, James Komack, John Erman, John Newland, Joseph Sargent, Lawrence Dobkin, Leo Penn, Michael O'Herlihy, Murray Golden Cast See All Writers D.C. Fontana, Jerome Bixby, Arthur Heinemann, David Gerrold, Jerry Sohl, Oliver Crawford, Robert Bloch, David P. Harmon, Don Ingalls, Paul Schneider, Shimon Wincelberg, Steven W. Carabatsos, Theodore Sturgeon, Jean Lisette Aroeste, Art Wallace, Adrian Spies, Barry Trivers, Don Mankiewicz, Edward J. Lakso, Fredric Brown, George Clayton Johnson, George F. Slavin, Gilbert Ralston, Harlan Ellison Franchise Star Trek Main Genre Sci-Fi Seasons 3 Producers Byron Haskin, Edward K. Milkis, Fred Freiberger, Gene L. Coon, John D. F. Black, John Meredyth Lucas, Robert H. Justman Creator Gene Roddenberry Story By gene roddenberry Streaming Service Paramount Plus Where To Watch Paramount Plus Executive Producer Gene Roddenberry Powered by Expand Collapse
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