This feature contains spoilers.
A crossover episode is an episode in which characters from one show cross over and appear in a different show. Sometimes a single story is written that crosses over both shows, beginning in an episode of the first show and concluding in the second. Other times, characters may appear in another show independently of their own. The second variety is often more fun, since it doesn’t require too much knowledge of the other show and therefore doesn’t alienate viewers who only watch one of the two.
Crossovers are, naturally, most common in spin-offs or the parent shows of spin-offs, and that’s where they tend to work best, especially in genre television. This is because, while it’s one thing for viewers of Friends to assume that the series takes place in the same version of New York City as Mad About You, it would be quite another for viewers of Farscape to imagine their favourite characters inhabiting the same universe as, for example, Battlestar Galactica. The rules for how the universe works are so different that crossovers between genre shows usually, though not absolutely always, stay within their own franchise.
A truly great crossover will work on two levels. For fans of only one show, the episode will offer a clear and self-contained story (no trying to force them to watch the second show with an unresolved cliffhanger) that’s enhanced by the presence of fresh characters. For fans of both, the episode will offer a bit of a shake-up that enriches both sets of characters. That’s a lot to ask and not all of these five episodes manage to hit both criteria – but nonetheless, they are five of the best.
5. Doctor Who, The Stolen Earth/Journey’s End
Crossed over: Doctor Who/Torchwood/The Sarah Jane Adventures
Plot crossover? No, the plot is essentially contained within main show Doctor Who’s continuity (though Torchwood fans who missed the season finale might wonder where Owen and Tosh are).
Why crossover? Although Russell T Davies’ final episodes of Doctor Who were the specials produced in 2009, this is really the culmination of his tenure as Doctor Who Head Guy. For his grand finale, he brought back all the (fairly numerous) companions from his four years on the show plus some memorable others and set them against the Daleks (obviously). This included Jack Harkness and Sarah Jane Smith, who came with their own companions from their own spin-off shows (also run by Davies).
Will viewers who stick to one show be confused? The inclusion of not only Harkness and Smith but the people who would, logically, be hanging out with them at the point it all goes pear-shaped adds to the realism of the episode and the sense of the show’s world as a complete one. It may mildly irritate anyone who hasn’t seen Torchwood or The Sarah Jane Adventures and doesn’t know who these extra people are, but it won’t substantially detract from their understanding the episode.
What’s great about it? The two episodes together provide some nice closure on the Russell T Davies years. Jackie and Mickey are back, Harriet Jones, Former Prime Minister sacrifices herself for her country (and everyone else’s countries too) and Rose gets left with a man who was made from the Tenth Doctor (this seems like the perfect solution to the problem of viewers wanting a satisfying end to their love story without the Doctor actually ending up with a wife or girlfriend or Billie Piper coming back, until someone points out that her new boyfriend is also half Donna). The scene where everyone operates the TARDIS together sums up the finale perfectly – self-indulgent, pretty cheesy, but oh-so-much-fun to watch. There is also a more serious point, concerning what travelling with the Doctor does to these people and how many of the people he meets end up dying for him, but although important, that can get somewhat lost in the nostalgia-fest. The main point is that, thanks to the crossovers, viewers get to spend some more time with all these beloved characters before the slate is wiped clean (and we say goodbye to them again, more briefly) in The End of Time.
Will it make me want to watch Torchwood or The Sarah Jane Adventures? If you aren’t watching already, probably not, unless you’ve seen publicity for Torchwood and want to know what happened to Owen and Tosh.
Quotable: ‘The man who abhors violence, never carrying a gun, but this is the truth, Doctor: you take ordinary people and you fashion them into weapons… behold your Children of Time, transformed into murderers. I made the Daleks, Doctor, you made this’ (Davros)
4. The X-Files, X-Cops
Crossed over: The X-Files/Cops
Plot crossover? I hope not.
Why crossover? Writer Vince Gilligan just really liked Cops.
Will viewers who stick to one show be confused? No – the set-up of Cops is pretty self-explanatory (I have to confess I’ve never seen an episode of Cops in my life, but I don’t think I’m missing anything in terms of following the episode).
What’s great about it? There’s a reason The X-Files appears twice on this list – the set-up of the show is so simple and relatively grounded that The X-Files can easily be crossed over with other shows, and since Mulder and Scully are FBI, they can travel anywhere within the United States and fit right in, which is why mixing up two such different shows actually works. For this episode plausibly to appear to be an episode of Cops, the monster had to be kept very much out of sight and shown by shadow and implication – and, as fans of Jaws are well aware, monsters are usually scarier when you can’t quite see them. It’s also refreshing to get an outsider’s view of Mulder and Scully as we see them through the film crew’s eyes. This episode is fairly light and a welcome change of pace, something much needed after the emotional crescendo of the preceding, seven-year-story-concluding Closure.
Will it make me want to watch Cops? No. Not unless you like that sort of thing, but in that case, you’re probably already watching already.
Quotable: ‘Mulder, have you noticed we’re on television?’ (Scully)
‘I don’t think it’s live television, Scully. She just said bleep!’ (Mulder)
3. Angel, Sanctuary
Crossed over: Angel/Buffy the Vampire Slayer
Plot crossover? ‘Sanctuary’ is the finale of a four-part story spread over both shows; This Year’s Girl and Who Are You? on Buffy followed by Five by Five and Sanctuary’on Angel. Faith is, of course, in all four episodes, but Sanctuary is the only one of the four to feature regular characters from more than one show, as Buffy turns up in LA, on Faith’s trail.
Why crossover? Angel’s first season included three crossovers, all featuring story threads begun in Buffy and concluded in Angel (though Sanctuary is followed up again in Buffy’s The Yoko Factor) in an attempt to entice viewers over from the parent show. The crossovers also maintain an healthy sense of realism in the series, given that Sunnydale doesn’t seem to be too far from LA, so there are times when you might expect our heroes to contact each other.
Will viewers who stick to one show be confused? Yes, very. Buffy viewers will be less confused – Faith just leaves town and never comes back, which could work as an ending in itself. Angel viewers, however, will have to catch up with Faith’s entire backstory from Buffy season three via the ‘previously on’ in addition to the events of This Year’s Girl and Who Are You? – and they’ll watch Buffy and Angel’s confrontation with no real idea why the two are so tense around each other. But then, there probably aren’t many any Angel fans who don’t also watch Buffy, so it’s a gamble worth taking.
What’s great about it? The whole four-part story is a very satisfying and well-written conclusion to Faith’s character arc, left largely unresolved as Buffy put her into a coma at the end of Buffy season three. But as a crossover, the most satisfying thing about Sanctuary is that Angel finally snaps and calls Buffy out on some of her behaviour. The first crossover (In the Dark) didn’t feature Buffy herself, while the second (I Will Remember You) focused on all the positive things about their relationship, as they spend a single day as a happy (and sexually active) couple before Angel has to go all noble on us and ruin it. But in Sanctuary, Angel snaps and points out some home truths to Buffy, like the fact that their break-up was hard on him too and he doesn’t have the option of hooking up with a sexy soldier to ease the pain. It’s very satisfying. (He goes back on it a bit in The Yoko Factor, but if you just watch Angel, you can ignore that).
Will it make me want to watch Buffy the Vampire Slayer? If you’ve stumbled onto Angel without seeing any Buffy, then yes, if for no other reason than to work out what on earth’s going on.
Quotable: ‘A cry for help is when you say “help” in a loud voice’ (Buffy, not sympathising with Faith)
‘You don’t know me anymore, so don’t come down here with your great new life and expect me to do things your way. Go home.’ (Angel to Buffy)
2. The Simpsons, The Springfield Files
Crossed over: The Simpsons/The X-Files/In Search Of…
Plot crossover? No.
Why crossover? Why do anything? Because it’s there? Because that gag about Mulder’s ID card was just too juicy to resist? Because this is The Simpsons, and at least 45% of the show runs on parody, and parody is much funnier when you can get the actual actors from the object of the parody involved.
Will viewers who stick to one show be confused? The episode is unlikely to be as funny or endearing if you’ve never seen The X-Files but having said that, the plot makes perfect sense (depending on your definition of ‘makes sense’) on its own.
What’s great about it? The Simpsons is a sitcom, and this episode is very funny. Considering all the crazy things that go on in Springfield, it’s only surprising Mulder and Scully didn’t turn up to investigate sooner. And Leonard Nimoy is in it, parodying a third show, because why parody one alien series when you can parody two? It also explains quite a lot about Mr Burns…
Will it make me want to watch The X-Files? Possibly, if you feel the need to see the genuine article that inspired Mulder’s two-dimensional swimsuit picture on his ID-card.
Quotable: ‘Hello. I’m Leonard Nimoy. The following tale of alien encounters is true. And by true, I mean false. It’s all lies. But they’re entertaining lies, and in the end, isn’t that the real truth? The answer is no’ (Leonard Nimoy)
‘I remember another gentle visitor from the heavens. Who came to earth, and then died, only to be brought back to life again. And his name was: E.T., the extra-terrestrial. I love that little guy’ (Reverend Lovejoy)
1. Star Trek: Deep Space Nine, Trials and Tribble-ations
Crossed over: Star Trek/Star Trek: Deep Space Nine
Plot crossover? Not exactly (either episode stands alone without viewers needing to have seen the other), but re-watching ‘The Trouble with Tribbles’ after seeing ‘Trials and Tribble-ations’ does give it a strange added dimension as you think to yourself ‘Sisko and Dax are up there looking for the bomb right now…’
Why crossover? Star Trek has done several crossover episodes over the years and both Deep Space Nine and Voyager did crossovers to celebrate the series’ 30th anniversary in 1996. Although Voyager’s (‘Flashback, in which we saw Tuvok’s earlier Starfleet career under Captain Sulu) was perfectly fine, it was ‘Trials and Tribble-ations’ that has become one of the most fondly remembered crossover episodes of all time.
What’s great about it? Star Trek has done several good crossover episodes (The Next Generation’s Scotty-starring ‘Relics’ stands out in particular). But this is the perfect tribute to Star Trek’s then-30-year history. The use of original footage (together with the same actor playing Darvin) provides a concrete, tangible link, not just with the stories and characters of the original series, but with the series itself. The 30th anniversary is, after all, a real-world celebration, not an in-universe one, so using genuine 1960s clips is a wonderful testament to the original series, while also providing an opportunity to reflect on how much things have changed over three decades. The technical combination of original footage with new is expertly done (though it’s a shame they couldn’t have found a less flirty bit of Kirk-footage for Sisko’s conversation with him…). Oh, and it’s also hilarious.
Will it make me want to watch Star Trek? Hopefully, this episode may have encouraged some younger viewers to seek out the original – although ‘The Trouble with Tribbles’ is undoubtedly one of the dafter episodes of the original series (though far from the daftest) it’s also one of the most fun, being built on a silly but simple premise and full of good humour. Sisko also explains the Bajoran Orbs to the Temporal Investigators, so viewers of the original series tuning in for the tribute won’t be lost either.
Quotable: ‘I’m a Doctor, not an historian’ (Bashir, channelling McCoy)
‘They are Klingons, and it is a long story… We do not discuss it with outsiders’ (Worf, on the Klingons’ appearance – the comedy is all in the delivery)
Honourable mention: It’s only a few seconds long, but Stargate SG-1’s 200th episode, ‘200’ features a very funny crossover with Farscape. The episode is built around resident alien Martin Lloyd’s attempts to come up with a film script based on Stargate Command, and features several short parodies of other classic series, including Star Trek and Gerry Anderson’s Supermarionation shows. It’s thanks to the presence of Ben Browder and Claudia Black that the Farscape parody included might be called a crossover, though only Black is playing her own character. OK, it rubs salt in the wound of Farscape’s cancellation in favour of SG-1, but it’s worth it for Thor-as-Rygel saying ponderously, ‘Yotz!’
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