For almost as long as there has been fiction there have been crossovers. Ancient Greek (and Roman) mythology is essentially one big expanded universe, full of crossovers (like Tiresias appearing in Oedipus Tyrannus and later in the Odyssey). The Illiad is essentially the Civil War (the comic, not the film) of the Ancient Greek world, the depiction of the Trojan War being primarily told in the Illiad, but plays and stories like Ajax, the Aeneid, The Trojan Women and then the Odyssey depicting satellite stories revolving around the War and it’s outcome.
The desire to see crossovers has continued in fiction from then on. Wherever there is a shared continuity (and even when there isn’t), you will find some sort of crossover. Comics thrive off of it; video games use it for mash-up vs. fighting games; films used them to pit your favourite horror villains against each other until Marvel Studios revolutionised the film industry by successfully launching the Marvel Cinematic Universe – a move which inspired apparently every single other franchise and studio to hurriedly copy suit and cause nearly every online film critic to vomit every time they see the words ‘shared cinematic universe’.
And then we come to TV. After comics, I’d hazard TV probably has the most crossovers of any medium. With cartoons in particular, where, much like comics, a lot of the popular franchises were owned or licensed by the same company, be it Hanna-Barbera, Cartoon Network, Warner Brothers or Disney. The DCAU is one of the most prominent of these, with what had started as Batman: The Animated Series is now a large enough connected universe to rival the comics they’re based on. But even before that, you would have Batman appearing on episodes with Scooby-Doo and Speed Buggy – all owned by Hanna-Barbera.
These are all good examples of the fun, cameo style type of crossovers. Even before nerdgasming was a thing, seeing heroes from different cartoons team up was exciting enough that I still remember it now. But crossovers are also found in the live-action side of TV and serve many purposes, as well as just for the fun of it.
Franchises like Star Trek have enjoyed longevity by using spin-offs to expand their universe. And where there are spin-offs, there are crossovers. With the exception of the prequel series of Enterprise and Discovery, every other tv series had a premiere with some sort of handover from the previous series (Bones appeared in Encounter at Farpoint, the Enterprise-D with Captain Picard literally delivered most of the crew to DS9, Voyager started its journey at DS9 – and while Enterprise didn’t start with a crossover, it ended on one … although more on that later). Buffy the Vampire Slayer incorporated clever crossover episodes with its spin-off Angel, although Buffy moving to a different network caused both to have independence from each other for most of their respective runs, which may have worked in their favour. Crossovers are also used to celebrate franchise anniversaries, like the very memorable ‘Forever Red’ episode of Power Rangers: Wild Force bringing together ten years of Red Rangers.
Marvel Studios revolutionised the film industry by successfully launching the Marvel Cinematic Universe – a move which inspired apparently every single other franchise and studio to hurriedly copy suit and cause nearly every online film critic to vomit every time they see the words ‘shared cinematic universe’.
Fiction is the grown-up way of make believe – and much like one might play with toys from different franchises, crossovers let creators do the same with beloved characters while also celebrating an expanded universe and long history, in a fun and nostalgic way. But are there downsides? Well, one criticism that occurs is sometimes there is too much crossover and references. The aforementioned ‘These Are The Voyages …’ episode of Enterprise is called out by fans and creators alike for focusing too much on Riker’s story, instead of what should have been a farewell to the characters of that actual show. On the flip side, the MCU’s presence on TV is often berated for not enjoying too much of a connection with the films, although a lot of this has to do with the films avoiding any references to the show, for bizarre reasons.
But this Goldilock’s-aspect of the right amount of crossover isn’t a flaw of crossovers – as with any and every TV trope, it all comes down to execution. A crossover between franchises, done with love for the story or franchise without compromising the integrity of the show usually without fail rewards long term fans for their loyalty and devotion to the characters and the universe.