The 11 best Star Trek in-universe crossover episodes

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Jun 16, 2017, 9:34 PM EDT (Updated)

Star Trek: The Original Series and its sequels built a world that was connected by an established history and in-universe decisions, the results of which you could see appear across each series airing at the time. While this helped link the TV shows, it wasn’t the only way the Star Trek series would connect together. Sometimes characters would cross from one show to another for appearances that would further tie the programs together and give us unforgettable crossover episodes.

Whether it was Klingons from The Original Series appearing on Deep Space Nine or a cameo by The Next Generation's Geordi La Forge on Voyager, the creative situations our characters often found themselves in during the science fiction franchise's run provided plenty of opportunities to bring familiar faces from other shows onto another program. Some cameos were more fleshed out than others and, like all types of episodes in the franchise, a few are more noteworthy than the rest. Here are the 11 in-universe crossover episodes I think stand out the most and deserve a second look all these years later.


"Through the Looking Glass," Deep Space Nine Season 3

This episode saw the mirror universe version of Miles O’Brien, known as Smiley, kidnap Benjamin Sisko and bring him to the parallel universe in order to finish the mission mirror Sisko was on before he was killed. Sisko agrees when he finds out the mission involves the mirror version of his wife Jennifer. Eventually Sisko succeeds and is able to return home. However, when he meets the other rebels, there's a recognizable face among the bunch from Voyager: Tuvok! He pops up when Rom and Bashir are discussing Sisko while heading to the space station, which is still called 'Terok Nor' here. Tuvok advises caution when Rom suggests throwing everything they have at the station. He's fittingly logical in his assessment of their situation. As Sisko and the rebels continue to plan, Tuvok interjects every once in a while.

Tuvok might not be given too much to work with here, but he's visible in all these group scenes and it's fun to see the character playing a role in the mirror universe. Considering he's stuck in the Delta Quadrant in the primary universe and wouldn't be able to interact with these characters anytime soon otherwise, it was a good idea to bring him into Deep Space Nine this way to show us what their interactions might be like.


"Timeless," Voyager Season 5

Like parallel universe episodes, episodes involving time travel and other timelines present another ideal opportunity for cameo appearances by actors from other series. This episode includes one such crossover. "Timeless" is a Voyager episode that also made my top 15 time travel list and focuses on a future Harry Kim and Chakotay communicating with a past Voyager in an attempt to keep the ship from crashing and killing everyone else on the crew. As they try to do this, they're pursued by Starfleet, who doesn’t want them meddling with the past, altering the timeline and changing the present.

Who should be the captain of the starship that ends up finding the former Voyager crew but Captain Geordi La Forge! From the starship Challenger, the familiar face from The Next Generation speaks with Chakotay and tries to convince him to talk face to face. La Forge knows what they're trying to do and since they can't let them succeed, he tells Chakotay about an offer from the Federation Council. They can't let him and Kim change the last 15 years and La Forge is thinking of protecting his current crew even if he might be doing the same thing in Chakotay's position. While Chakotay constantly turns him down, La Forge contacts the shuttle again and tells them to lower their shields so they can be beamed out safely when a warp core breach is imminent . Chakotay refuses and just tells him to get to a safe distance.

Even though it's only for a few brief scenes, it's great to see La Forge as a captain here, trying his best to fulfill orders while also understanding where the two Voyager crew members are coming from.


"The Forsaken," Deep Space Nine Season 1

A group of Federation ambassadors arrives on the station ... and who should be among them but Lwaxana Troi from The Next Generation! When Odo helps return something stolen from Troi, he immediately sparks her interest. She starts to flirt with Odo, who complains about her behavior, but Sisko doesn't seem to see her as harassing him at all. This results in Odo and Troi ending up stuck in an elevator when a probe the station picked up starts messing with their computer.

Their time in the elevator is where Troi's appearance here shines. She starts telling Odo stories to fill the silence, though Odo doesn’t show any interest in returning the favor. Troi gets him to eventually discuss some things and as the time passes, Odo finds himself reaching the 16-hour mark where he needs to turn into a liquid. It leads to a very vulnerable moment for Odo because no one has ever seen him like this before. Troi offers her wig to him, admitting no one's seen her like that, and when he turns into liquid, he settles into her lap.

From Troi and Quark arguing in the beginning to Odo and Troi saying goodbye at the end, this episode really lets Troi stand out during her visit to Deep Space Nine. It quickly goes from her showing her typical behavior to showing her true self to Odo in the elevator, where he's forced to do the same. A fantastic and unique rapport develops between the two very different characters, which helps us learn more about them and see them in a new light.


"Defiant," Deep Space Nine Season 3

In this episode, a man who appears to be The Next Generation's William Riker is paying a visit to Deep Space Nine, but when Kira gives him a tour of the station and the Defiant, he stuns her and steals the ship. He turns out to be Thomas Riker, Will's copy from a transporter accident and a member of the Marquis. Sisko helps the Cardassians track him down while Kira tries to sabotage the ship and argues with Riker about his plans, though he doesn't stop until they are eventually caught. While everyone else will be taken to a Federation ship unharmed, Riker must pay the price for his actions and spend the rest of his life in a Cardassian labor camp, and Kira convinces him to take the deal for his crew.

It's an interesting twist having the character be Thomas instead of Will here, and he's got a surprisingly great dynamic with Kira throughout the episode. From their flirty banter to their heated arguments, their chemistry made for an interesting relationship that would have been cool to see explored in an additional episode. As it is, this was Thomas' only appearance on Deep Space Nine, but at least it's a memorable one!


"Pathfinder," Voyager Season 6

The Next Generation's Reginald Barclay and Deanna Troi feature in this episode, which starts with the two namedropping some other members of the Enterprise crew before getting to the heart of the story where we find out Barclay is dealing with a Voyager obsession. It began with his work on the Pathfinder project, which is trying to open communications with the starship. To work on one of his ideas, he enters a holoprogram of Voyager and keeps returning. After he's kicked off the project, Barclay tells Troi that leaving the Enterprise was like losing his family, but Troi tells him the holo-Voyager can't replace that since they're not real. Despite everything, he goes against orders and ends up succeeding in sending a message to Voyager from Earth. Starfleet and Voyager end up talking in an emotional scene and it sets the stage for future contact between the two.

It’s entertaining to see Barclay and Troi interact so much in this episode, where most of the versions of the Voyager crew we see are all holographic! How this holographic crew interacts with Barclay is also quite amusing, as he's able to be very social with them and is, as usual, more comfortable in a holoprogram than in real life. The episode presented a unique way to bring these Enterprise characters into the fold for Voyager without needing time travel or an alternate reality. It showed us what these old characters are up to at this point in time and how those on Earth have not forgotten about the Voyager crew.


"Relics," The Next Generation Season 6

After 75 years of being stuck in a transporter, The Next Generation crew frees Scotty from The Original Series. Forced to adjust to how things have changed over the years, he peppers Geordi La Forge with his many questions and offers him lots of advice from his own experiences. While Scotty is encouraged to rest, he finds it hard to stay away from work. Eventually he gets in the way and La Forge snaps and tells him so. After a visit to Ten Forward, Scotty goes to the holodeck and recreates the bridge of his Enterprise. Picard shows up and they're able to talk about what Scotty's going through, causing Picard to ask La Forge to help make Scotty feel useful. Ultimately the two working together saves the day, and in the end Scotty acquires a shuttle to use for retirement and/or future adventures.

Things may have changed over time but Scotty is still the same Montgomery Scott we remember from The Original Series. Seeing him miss the old ship and crew really tugs at your heart and makes you long for the show that started it all. It's great to see him in action with the current Enterprise crew and especially fun to see him bond with La Forge.


"Flashback," Voyager Season 3

Three characters from The Original Series show up in this Voyager episode. It all starts with Tuvok seeing an anomaly that Voyager goes to investigate. He begins to shake and become disoriented. On the way to sickbay he seems to relive a memory, but it's not a situation he remembers. Ultimately Captain Janeway mind-melds with Tuvok to try and deal with what appears to be a repressed memory.

During this, they arrive on the starship Excelsior where Tuvok was a junior science officer under the command of Captain Sulu during the events of the movie Star Trek VI: The Undiscovered Country. Along with Sulu, Commander Janice Rand is also aboard the starship. They end up heading to a nebula that looks very similar to the one Voyager approached as Sulu decides to try to rescue Kirk and McCoy. Of course Tuvok objects, leading to a great scene with Sulu and Rand. Another actor from the first show hails them along the way, the Klingon Kang. While reliving these events, the Voyager crew is able to figure out what is happening to Tuvok and both the Vulcan and Janeway survive.

Sulu and Rand are both given a decent amount of face time in this episode and have some awesome scenes with Tuvok. From the serious to the funny, it's a really enjoyable episode. It's awesome seeing Sulu in command, seeing Rand play a key role and having Kang pop up in an unexpected cameo.


"Blood Oath," Deep Space Nine Season 2

Three familiar Klingons from The Original Series, Kang, Koloth and Kor, visit the station in this Deep Space Nine episode. When Dax hears that Kor is in a holding cell, she goes and tells him she used to be his old friend Curzon. They meet Koloth and Kang, who announces he has found the Albino. It turns out Curzon was the godfather to Kang's son, who was named Dax, and he took a blood oath to avenge his death along with the sons of Koloth and Kor, who were murdered by the Albino.

Dax struggles with whether to stick to Curzon's oath, even though Kang says she's free of it, and ultimately decides she has to go. She speaks with Kor, fights Koloth in a holosuite and convinces Kang to let her go with them after angering him. Dax realizes Kang plans to die and agreed to the Albino's invitation for an honorable battle. Dax persuades him to try and win, leading to some great Klingon combat scenes. While they are able to kill the Albino, both Kang and Koloth are lost in the battle.

It’s always interesting to see Dax’s former lives explored, and this episode presented the perfect opportunity to bring back some familiar Klingons from The Original Series. It's great seeing how these warriors have aged and are dealing with that, as well as how they respond to Jadzia. Learning about their shared pasts offered insight into the larger history of the franchise as well as revealed the histories of these characters on a personal level. It was a very creative and plausible way to connect the two different eras, giving us one of the best crossover episodes of Star Trek.


"Sarek," The Next Generation Season 3

Here the Enterprise is hosting the first meeting between the Federation and the Lagarans. They pick up Spock's father Sarek from The Original Series to act as ambassador, and it's rumored he will retire after this last mission. It becomes clear that something is up with Sarek during the voyage as he sheds tears during a musical performance and the crew starts to experience bouts of extreme anger that only started once Sarek came aboard. They determine Sarek must have Bendii syndrome and Captain Picard confronts Sarek about it, leading to an emotional scene and an intense one-on-one conversation between the two. Eventually the Captain goes through a mind-meld with Sarek to help him control his emotions during the negotiations.

Patrick Stewart and Mark Lenard are excellent in this episode. They handle the emotional scenes beautifully, and how they play off each other is perfect.


"Unification," The Next Generation Season 5

Sarek returns in this two-part episode that also sees The Original Series' Spock making a visit to The Next Generation. It all starts when Captain Picard is told Ambassador Spock has been spotted on Romulus. He disappeared without warning and there are concerns he may have defected. Picard heads to Vulcan to see if Sarek knows anything and finds him very sick with his emotions taking over. He is able to eventually speak with Picard and mentions a Romulan senator Spock may have went to see. With a Klingon ship, Picard and Data cross the neutral zone and head to Romulus, along the way learning that Sarek has died.

Once on Romulus they find Spock, who explains he's there on a personal mission to help those Romulans interested in learning about Vulcan and see if reunification between the two planets is possible. Unfortunately it turns out the proconsul Spock is dealing with is not interested in reunification but conquest. In the end they're able to stop a Romulan invasion force from making its way to Vulcan, but Spock still refuses to leave the planet. He is determined to stay behind and continue to help.

Mark Lenard is again brilliant here in his role as Sarek. This time he must portray the beloved character as even more ill and he plays the scene with Picard perfectly. His discussion about Spock with Picard is a fitting last way to see the character in the franchise .. and the perfect way to bring Spock into the fold, taking part in some 'cowboy diplomacy' that Kirk would no doubt approve of. He’s still the same Spock, logical but with just enough emotion, and scenes such as the last one where Spock mind-melds with Picard to feel his father's thoughts is played beautifully by Leonard Nimoy. It all results in an amazing and unforgettable two-parter.


"Trials and Tribble-ations," Deep Space Nine Season 5

The beloved TOS episode "The Trouble with Tribbles" is revisited in this Deep Space Nine episode involving time travel. The Klingon spy Darvin shows up in the present and takes the crew back in time to try to change the events that took place on the K7 deep space station. This results in the Deep Space Nine cast having to blend in to the time period, dressing in the old uniforms and trying to investigate while avoiding too much contact with anyone. They don't completely get away with not interacting with the original Enterprise crew, though, leading to some awesome moments where it looks like Kirk and the others really are speaking with the current cast. They ultimately catch Darvin and return to the present, leaving the timeline intact.

Other than an actual appearance by the present-day Darvin, this melding with The Original Series may mostly be thanks to technology, but "Trials and Tribble-ations" is still the best crossover in the entire TV franchise. It brilliantly mixes the two generations of Star Trek in an impressively seamless way, where it really does look like the two casts are interacting in many of the scenes. Plus "The Trouble with Tribbles" is full of so much to play with thanks to its humor and drama that this follow up episode has it all. There are high stakes and tons of amazing moments. We get a sense of Dax's history in this period, witness a hilarious discussion between Odo and Worf about tribbles, see O'Brien and Bashir questioned by Kirk and so much more. It even ends fittingly with the tribbles making their way into the present and breeding on the station! "Trials and Tribble-ations" truly is everything a crossover episode should be.

Which crossover episode between Star Trek TV series is your favorite? Tell us in the comments!