The Best Sci-Fi Crossovers That May Have Slipped Under Your Radar

Science fiction is a genre with a multitude of subgenres, giving it perhaps the most extensive reach of any in popular culture. Some of the most beloved films, comics, and television series are firmly set within the realm of sci-fi, many with a massive following of fans. Franchises like "Star Wars," "Star Trek," and others often have followers numbering in the tens of millions... if not more.

Because sci-fi is such a popular genre, companies have been merging them for some time, creating unique crossover events in all forms of media. Every once in a while, you'll see the crew of the USS Enterprise teaming up with the Doctor in his TARDIS or the X-Men in the pages of Marvel Comics. Whatever the medium, there are examples of science fiction franchises fraternizing, and fans lined up to fantasize over the possibilities.

Often, these kinds of crossovers are equal parts fan service and outrageous storylines, so they don't always make a ton of sense. But that rarely matters when fans get more of what they love, and that's pretty much what a good sci-fi crossover is — a combination of IPs in a fun, exciting new setting. Below, arguably the best to ever see the light of day.

"Star Trek" is one of the most beloved science fiction franchises ever created, so having it crossover with another high-profile franchise makes sense. That said, one that most fans likely never expected to see "The Original Series" cast of characters exist alongside is probably "Planet of the Apes." Like "Trek," "Planet of the Apes" is an expansive franchise spanning nearly a dozen films, a television series, books, and comics — which is where this particular crossover takes place, thanks to IDW Comics and Boom! Studios.

The 2014 series "Star Trek/Planet of the Apes: The Prime Directive" is a crossover collaboration published by Boom1 and IDW. The limited series covers five issues that feature the debut artwork of talented illustrator Rachael Stott. The crossover takes characters from each franchise and throws them together via a portal between universes. A Klingon vessel and the USS Enterprise travel through the portal to arrive in the "Planet of the Apes" universe.

Once there, Captain James T. Kirk and company work to stop the Klingons from enacting their plan to install a puppet gorilla government on Earth. The only thing standing in their way is the Prime Directive. Still, with the aid of the chimpanzees and George Taylor, they stop the Klingon plan. Cornelius, Milo, and Zira are left in orbit with a tricorder and a plan to transport them back in time using the same trick the Enterprise crew pulled off in "Star Trek IV: The Voyage Home."

"The X-Files" was a steady performer on Fox throughout the 1990s, and it crossed over with several other series, including "The Simpsons," "Cops," and more. The show also featured a crossover with "Millennium," a somewhat similar series canceled too soon, having run from 1996 to '99. When the show ended, it came suddenly, leaving some open stories that left fans wanting more. The "X-Files" stepped in to save the day or, at the very least, offer some closure.

In "The X-Files" seventh season, the episode "Millennium" served two purposes. It featured a crossover with the series "Millennium," and it closed the story for Frank Black, the series' protagonist played by Lance Henriksen. In "Millennium," Black worked as a freelance forensic profiler who was once an FBI Agent. He has the unique ability to see through the eyes of murderers and serial killers, which comes in handy when investigating that sort of thing.

"Millennium" features Agents Mulder and Scully investigating a previously-occupied grave. They turn their investigation to the now-dissolved Millennium Group, taking them to Frank Black, who is undergoing treatment at a mental institution. He's reluctant to help but comes around and gives them assistance. There are zombies, an end-of-the-world millennium threat, and many near-death experiences. In the end, Black decides to check himself out of the mental institution and the clocks strike midnight, ringing in the new year with the apocalypse notably absent.

Steven Spielberg and George Lucas have been friends for decades, and they've worked together on multiple (typically, hugely successful) projects. Their collaborations have led to more than a few crossovers and friendly homages to one another, which is why you can find R2-D2 and C-3PO in some hieroglyphics during a scene in "Raiders of the Lost Ark." Of course, the directors don't limit their crossovers to movies they work on together, so you can find two prominent ones in "E.T.: The Extra-Terrestrial" and "Star Wars: Episode I — The Phantom Menace."

In "E.T.," during the Halloween scene with the children dressing up their alien friend as a ghost (and pretending he's Gertie), there are several characters wearing costumes. As E.T. peers through the holes in his costume/sheet, he notices some notable characters, lingering on one in particular. A child approaches dressed as Jedi Grand Master Yoda, and the camera stays on this kid long enough for any fan to register that this movie is set in a world where "Star Wars" exists. 

Decades later, Lucas took things a step further with "The Phantom Menace." Numerous alien species can be seen participating in the background during the scenes involving the Galactic Senate, and some are close-up, while others are a bit further away. When Queen Padmé Amidala calls for a no-confidence vote on the chancellor, numerous senators cheer or express their outrage. One group of representatives is made up of the same species as E.T., which the Wookiepedia calls "Asogians." 

While Godzilla is often thought of as the original kaiju, King Kong helped establish the concept of a monsterverse decades earlier. The success of "King Kong" proved audiences wanted to see giant monsters destroying cities, and after the atomic bombings of Japanese cities in WWII, the "Godzilla" franchise was born. The two franchises had some similarities, so fans often argued over who would win in a fight, King Kong or Godzilla. To answer that question, "King Kong vs. Godzilla" was released in 1962, and then a new generation sought the answer with "Godzilla vs. Kong" in 2021.

For "King Kong vs. Godzilla," the two titans come into contact with one another when an American submarine awakens Godzilla. This happens when the sub and a pharmaceutical company capture Kong for a promotional campaign. This activity ultimately results in a fight between Godzilla and Kong on Mount Fuji. Godzilla takes the lead in their battle, but Kong is struck by lightning, supercharging him. They trade blows and fall off a cliff into the water, continuing their fight. When it's done, only Kong surfaces and begins a long swim back to his island.

Nearly 60 years later, a new film came along to pit the world's biggest monsters against one another. "Godzilla vs. Kong" brought their fight to the new millennium, and it's brutal. As they battle throughout Hong Kong (damaging much of the city in the process), Godzilla takes the lead. Kong is revived with some human help, and he joins Godzilla in fighting Mechagodzilla. When the battle is over, Kong and Godzilla go their separate ways.

There's an unwritten rule in comics covering crossovers: When unfamiliar heroes meet for the first time, they must fight. 

This is typically followed by a realization that everyone is on the same side; they then agree to work together, and everyone walks away friends ... except for the bad guy, of course. It seems that rule made its way onto the "Batman" television show from the 1960s, because that's pretty much what happens in "A Piece of the Action," which features a guest appearance by the Green Hornet and Kato.

The episode revolves around a counterfeiting scheme that draws the Green Hornet and Kato to Gotham City. The Dynamic Duo is already on the case, and Batman and Robin quickly misidentify their fellow heroes as villains. A fight ensues, and various minions jump into the fray as the two teams of heroes trade punches and kicks. Ultimately, they realize that they're on the same side, and things calm down.

The heroes band together and do what heroes do — they beat the bad guy. When this episode aired, "Batman" was at the height of its popularity while spinoff "The Green Hornet" was fading despite the fact that Bruce Lee, who terrified Burt Ward during filming, was involved. Unfortunately, the crossover episode failed to revive interest in "The Green Hornet," and the series was canceled that same year. "Batman" lasted another season, and today has maintained quite a bit more relevance in popular culture. Decades later, they met again.

"Futurama" wastes no time getting to numerous "Star Trek" references in its pilot episode. There are automatic doors, and in the Head Museum, Jonathan Frakes guest stars for a single line shortly after Leonard Nimoy makes an appearance. It's almost impossible to separate "Star Trek" from "Futurama," but that doesn't mean one tolerated the other. In the episode "Where No Fan Has Gone Before," it's revealed that Trekkies grew into a religion, and it got so bad they were sacrificed in a way befitting their often-perceived virginal nature — they were thrown into a volcano.

Despite this, Fry loves "Star Trek," and he takes Nimoy's head on a mission to recover the only "Star Trek" tapes still in existence. The Planet Express crew crashes on Omega 3, where they're greeted by the original cast of "Star Trek," and a bunch of memorabilia from the show. They also meet Melllvar, an energy entity who is a die-hard fan of the series who gave the cast new bodies. He forces everyone into a mock "Star Trek" convention (which the actors absolutely loathe).

Eventually, Melllvar pits the two crews against one another, and there are some interesting engagements. Ultimately, they work together to rebuild the Planet Express ship so they can escape. Melllvar chases them with a Klingon Bird of Prey, and he's left behind, agreeing that he cannot obsess so much about "Star Trek." Of course, Fry learns the same lesson, and the Planet Express crew takes the "Star Trek" cast back to Earth.

"Ready Player One" is a 2011 sci-fi novel from Ernest Cline, then adapted by Spielberg into a 2018 movie; it's about a kid who goes by the name Parzival, who joins a worldwide Easter egg hunt in a virtual universe called the OASIS. The hunt revolves around the OASIS' creator, James Halliday, and his love of popular culture from the 1970s, '80s, and '90s. When Parzival is inside the OASIS, there isn't a pixel that doesn't relate to one franchise or another. Essentially, "Ready Player One" is a massive homage to the genre, and it accomplishes this by crossing over virtually every franchise in existence.

You can watch the Spielberg movie a hundred times and spot something new with each viewing — it's that packed with content. Still, several notable depictions cameos can't go by unnoticed. These include the "Star Trek"-themed funeral for Halliday, Speed Racer's car (the Mach 5), Parzival's ride, which is the DeLorean from "Back to the Future," the motorcycle from "Akira," and there are tons of Stormtroopers, R2-D2, and other characters from various "Star Wars" films.

"Ready Player One" is a love letter to the many sci-fi franchises that came before it. As it happens, the movie is very different from the book, though structurally the same. Many of the pop culture elements were changed to relate to a broader audience, familiar with more contemporary examples. This effectively makes the homages and references in the film almost entirely different from the Cline novel.