The X-Men nor the various crews of the Enterprise have crossed over with plenty of other pop culture franchises. Frequently fighting with and alongside heroes of their own universe, the X-Men have also crossed universes to meet the Teen Titans and members of the Justice League. The crew members of Star Trek have similarly rubbed shoulders with the Green Lantern Corps, Planet of the Apes and even Doctor Who's Eleventh Doctor.
The worlds of Star Trek and X-Men have met three times, and the stories have gotten stranger with each successive meeting. The X-Men and the crew of the Enterprise first crossed paths in 1996's Star Trek/X-Men. Captain Kirk encountered the X-Men when Marvel's mutants accidentally fell through a portal to an alternate reality during a spacefaring mission. The story found Kirk and the X-Men fighting a resurrected Gary Mitchell and the mutant Proteus. This crossover - infamous for its fight between Wolverine and Spock - may have been a bit bizarre, but it was nothing compared to the sequels that involved the The Next Generation Enterprise crew.
Star Trek: The Next Generation/X-Men: Second Contact (by Dan Abnett, Ian Edginton, Cary Nord and Scott Koblish) is a direct sequel to the film Star Trek: First Contact, and was published in 1998. Picking up where the movie left off, it finds the crew of the Enterprise-E attempting to return home following their battle with the Borg. Creating a displacement field to return to their own time, the Enterprise emerges in an alternate reality of the 1990s, orbiting the X-Men's Earth. While attempting to repair their technology, the crew encounter the X-Men - beaming down directly into the X-Men's mansion. After a brief fight - in which Data easily fends off both Wolverine and Colossus - the two groups decide to team up.
With the Enterprise's presence causing a rift between worlds, the two groups head back in time, where events from both universes' history have been drastically altered. This sees members of the Enterprise popping up throughout Marvel history (including Tasha Yar, as a freedom fighter during "Days of Future Past") and the X-Men participating in major Star Trek events, most notably "The Battle of Wolf 359."
Michael Jan Friedman's Planet X is the prose sequel to Second Contact, published in 1998. The people of the planet Xhaldia are suddenly mutating - changing physically, and developing strange new powers. When Picard and his crew are sent to defuse the situation, they once again encounter the X-Men, who are later revealed to have been pulled into this universe by a meddling Q.
With the X-Men on the Enterprise, the two teams bond and enjoy each other's company. Friedman hints at a mutual attraction between Picard and Storm, who share a cup of tea and discuss the pressures of leadership. Worf and Wolverine battle foes on the holodeck before sharing a few glasses of Prune juice (Worf's favorite drink) together. The longer, more in-depth structure of a novel allows Friedman to go deeper than most comic books. While crossovers like Star Trek and the X-Men are nothing new - especially not during the '90s - it is unusual to have comic book characters like the X-Men appear in a Star Trek novel. With less focus on action, Planet X allows its characters to just hang out and have fun.
In the story's most curious touch, Friedman notes the physical similarities to Picard and Professor Xavier, with characters remarking that the pair bear a strong resemblance to one another. This is particularly prophetic, given that Planet X pre-dates 2000's X-Men movie by two years. While Picard and Xavier never really meet, the novel ends with the Enterprise being fitted with a holographic representation of the Professor that Picard can chat with at will.
While the 90s were a peak time for comic book crossovers, those of The Next Generation and the X-Men are two of the era's strangest. Neither Second Contact nor Planet X panders to unfamiliar audiences, as appreciating the first book requires a fairly extensive knowledge of both Star Trek and X-Men history. The finale of Second Contact is particularly convoluted, calling upon Wesley Crusher and The Traveler, culminating with a pitched battle against Kang the Conqueror, the Borg and the Sentinels. Planet X sees the Mutants on the Enterprise crew's home turf, in more ways than one. Star Trek tie-in novels are an incredibly popular institution, and Friedman takes it very seriously, giving readers the best of both worlds.
In the Federation, the X-Men find hope - seeing a future that celebrates the differences of others, whether they are mutant, human or alien. In the X-Men, the crew of the Enterprise meet powerful new friends and allies. This is a unique melting pot of both universes' sensibilities, resulting in two of the strangest crossovers either team has ever witnessed.