Several characters within the Star Trek franchise, primary and secondary, often made crossover appearances between one series and another. This included appearances of established characters on premiere episodes of new series, a few long-term transfers from one series to another, and even crossovers between Trek films and television. A few crossover appearances, such as that of Spock on The Next Generation and the time-travel of the crew of Deep Space Nine to the era of The Original Series were especially lauded by both fans and critics.
Appearances in series premieres After The Original Series, at least one character from an earlier series appeared in the premiere episode of each new series. These were the appearances of most of the Original Series main cast in "Beyond the Farthest Star", the first episode of The Animated Series; Leonard McCoy in "Encounter at Farpoint", the first episode of The Next Generation; Captain Jean-Luc Picard and Miles O'Brien in "Emissary", the first episode of Deep Space Nine; Quark and Morn in "Caretaker", the premiere of Voyager; Zefram Cochrane (from the original series episode "Metamorphosis" and the film Star Trek: First Contact) in "Broken Bow", the premiere of Enterprise; Sarek in "The Vulcan Hello", the first episode of Discovery; Sylvia Tilly in "Runaway", the first episode of Short Treks; Jean-Luc Picard and Data in "Remembrance", the first episode of Picard; NOMAD (from the original series episode "The Changeling") in "Second Contact", the premiere of Lower Decks; a training hologram resembling Captain Kathryn Janeway in "Lost and Found, Part 1", the first episode of Prodigy; and Captain Christopher Pike, Spock, Una, Christine Chapel, Nyota Uhura, Joseph M'Benga, Robert April, T'Pring, and George Samuel Kirk in "Strange New Worlds", the premiere of Strange New Worlds.
Long-term transitions Two long-term transitions were the transfer of Worf and Miles O'Brien from permanent characters on The Next Generation to permanent characters on Deep Space Nine. Worf stayed on The Next Generation until its conclusion, then transferred from the start of season 4 of DS9. Chief O'Brien had appeared in fifty episodes of The Next Generation, but only gained opening credits billing on DS9, in which he appeared in almost every episode of all seven seasons. While not seen as often as Chief O'Brien, his wife Keiko O'Brien also transitioned as a series regular guest star from The Next Generation to Deep Space Nine, bringing their daughter Molly. Miles O'Brien reprised his Next Generation role in that series' finale "All Good Things...".
Episodes and films focused on crossovers
Television-to-television crossover
An especially significant crossover is Spock's appearance on the two-part Next Generation episode "Unification". Spock meets Data and they exchange opinions on the relative value of logic and emotion, and share mutual impressions of Next Generations Captain Picard, who for Spock is a model of logical behavior, and for Data is a model of what it means to be human. Critics such as Ina Rae Hark have noted this encounter between two non-human characters in which they summarize their contrasting attitudes to humanity. Data embodies Spock's ideal of pure logic, even as he aspires to become more human.
Scotty appears in TNG Season 6, Episode 4 "Relics".
Recycled footage from the original series episode "The Trouble with Tribbles" is used extensively in Deep Space Nines time-travel episode "Trials and Tribble-ations". In addition to extensive archival footage of cast from the original series, actor Charlie Brill (portraying Klingon spy Arne Darvin) appears in both new and archival footage. The episode was designed to coincide with the 30th anniversary of the original series. Critic Matthew Kappell notes that the new episode simultaneously "parodies and valorizes" the original series, highlighting the discontinuities between two eras of Trek while trying simultaneously to weave them together.
William Riker and Deanna Troi from The Next Generation appear in a flash-forward to the future in "These Are the Voyages...", the final episode of Enterprise. This episode was widely criticized by both cast members and fans for playing more like a Next Generation episode to the point of being an inappropriate wrap-up for the series.
Strange New Worlds episode "Those Old Scientists" features live-action appearances of Beckett Mariner and Brad Boimler from the animated Lower Decks series.
Film-to-television crossover
In the Voyager episode "Flashback", events from the film Star Trek VI: The Undiscovered Country (1991) play a pivotal role, and are reenacted with the same actors, thus allowing George Takei to reprise his role as Hikaru Sulu. The episode contains the added revelation that the character of Tuvok was on Sulu's ship Excelsior at the time. Critic Lincoln Geraghty cites this as an example of Star Treks ongoing propensity for reverential recognition of earlier versions of the series. In a bit of rather glaring discontinuity, the character of Lieutenant Dimitri Valtane is shown to be both alive at the end of the film and dead in the episode. Actor Tim Russ, who portrayed Tuvok, also made an appearance as a bridge officer from the same fictional time period, serving on the Enterprise-B in the feature film Star Trek Generations (1994).
Television-to-film crossover
The film Star Trek Generations brought Kirk and Picard, Enterprise captains from different centuries, together to defeat a common enemy. It also marked the passing of the film franchise from the original series cast to that of The Next Generation.
Characters more prominent on subsequent series A few occasional recurring characters introduced in one series continued into other series, sometimes attaining more significant roles in their subsequent Trek series than in the one in which they were first introduced:
Spock's father Sarek appeared in only one episode of the original series, but became a more developed character in three movies and appeared in the Next Generation episode named after him and the first part of "Unification". Sarek's appearance in the former is described in The Star Trek: The Next Generation Companion as "the first major unifying event tying together the old and new Trek eras since McCoy's cameo" in the original series. Sarek later appeared in Star Trek: Discovery. Recurring character Q, originally introduced in the premiere episode of Next Generation, continued to appear in episodes of Deep Space Nine, Voyager, Lower Decks and Picard. Teleplay writer Robert Wolfe found it difficult to incorporate Q into the DS9 universe as the character had been conceived so much as a foil for Captain Picard. He eventually decided to use Q to show personality differences between Picard and Sisko. It was also difficult to decide how to introduce Q into Voyager, as it was necessary to explain why Q did not return the ship to Federation territory. Although appearing in only five episodes of The Next Generation and six episodes of Voyager, the character of Reginald Barclay became quite pivotal to the story-arc of the final season of Voyager, significantly contributing to the reestablishment of contact between the ship Voyager and the Federation. Barclay also appears in a cameo role in the eighth movie in the film series, Star Trek: First Contact. Three Klingon characters from separate individual episodes of the original series, Kor, Koloth and Kang, all appear in the Deep Space Nine episode "Blood Oath", and Kor further appeared in DS9 episodes "The Sword of Kahless" and "Once More Unto the Breach". Crew members who worked on "Blood Oath" felt that a "special connection" was being made to the original series. Kang made an additional cameo appearance on Voyager, verbally dueling with Captain Sulu in a brief scene of the episode "Flashback". The two Ferengi, Arridor and Kol, were minor characters in the TNG episode "The Price" but were the principal antagonists in the Voyager episode "False Profits". The Borg race appeared in six of the Star Trek series. Introduced in The Next Generation, they would appear in Deep Space Nine, Voyager, Enterprise, Picard, and Prodigy. Their role in Deep Space Nine was limited to an appearance in the premiere episode to explore Benjamin Sisko's troubled past (his wife having been killed in a major confrontation with the Borg), while their role in the Enterprise episode "Regeneration" served as a 'sequel' to Star Trek: First Contact, as an archaeological expedition discovers the remains of the time-travelling Borg sphere that was destroyed in First Contact and unwittingly reactivate a pair of Borg drones. However, the Borg became a recurring threat on Voyager. One crossover character was the Borg Queen first introduced in the Next Generation feature film Star Trek: First Contact. She would then become a recurring character on Voyager appearing in the two-part episodes "Dark Frontier" and "Unimatrix Zero" and in the series finale "Endgame". These episodes form a narrative thread in which the Borg Queen is battling with Janeway in attempts to re-assimilate Seven of Nine. Only in the final Voyager episode is the Borg Queen played by the same actress who played her in the feature film Star Trek: First Contact, Alice Krige. The magazine Cinefantastique described Krige's appearance as the highlight of this episode.Cinefantastique: Volume 36 2004 Eventually a new, cloaked form of the Borg Queen appears in Star Trek: Picard, quickly followed by a version more reminiscent of the appearance and personality of the character seen in First Contact and Voyager, in an alternate timeline. Another crossover character is Hugh, a Borg drone who appeared in the TNG episode I, Borg and subsequently in the first season of Picard.
All crossovers
Character crossovers The Original Series on The Animated Series
James T Kirk, Spock, Montgomery Scott, Hikaru Sulu, Nyota Uhura and Christine Chapel all appear as part of the voice cast. The robots of Alice and the White Rabbit from Lewis Carroll's 1865 novel Alice's Adventures in Wonderland reappear from the episode "Shore Leave" in the episode "Once Upon a Planet" The Guardian of Forever, Sarek, and Amanda Grayson appear the episode "Yesteryear" Cyrano Jones, Koloth, and Korax appear in the episode "More Tribbles, More Troubles" Kor appears in the episode "The Time Trap" Harry Mudd appears in "Mudd's Passion" Bob Wesley appears in the episode "One of Our Planets is Missing" after last appearing in "The Ultimate Computer" The Original Series on The Next Generation
Leonard McCoy appears in the episode "Encounter at Farpoint " Sarek, Spock's father, appears in the episode "Sarek" and again in "Unification I" Spock appears in both parts of the episode "Unification" Montgomery Scott appears in the episode "Relics" The Original Series on The Next Generation series of filmsJames T. Kirk, Montgomery Scott and Pavel Chekov appear in the film Star Trek Generations Zefram Cochrane (originally played by Glenn Corbett) appeared in the film Star Trek: First Contact played by James Cromwell
The Original Series on Deep Space Nine Kang, Koloth, and Kor (with their cranial ridges "restored") appear in "Blood Oath". Kor appears subsequently in "The Sword of Kahless" and "Once More Unto The Breach". James T. Kirk, Spock, Leonard McCoy, Montgomery Scott, Pavel Chekov, Nyota Uhura, Cyrano Jones, and Korax appear in "Trials and Tribble-ations" (There were clips from the Star Trek: The Original Series episode "The Trouble with Tribbles ", but Benjamin Sisko 's conversation with James T. Kirk was based on footage taken from the episode "Mirror, Mirror"). The antagonist Arne Darvin appears through both archival footage, and additional scenes by an older Charlie Brill
The Original Series on Voyager Hikaru Sulu, Janice Rand and Kang appear in the episode "Flashback"