It may sound strange, but the X-Men and the original crew of Star Trek really did come together for a glorious moment, united to defeat a mutual enemy. Neither team are strangers to inter-dimensional travel, or encountering unexplained phenomenon. And for that reason, their comic crossover isn't as crazy as many would actually expect.
The issue: Star Trek/X-Men from Scott Lobdell and Marc Silvestri. Spatial rifts and sentient magnetic fields do all the heavy lifting to get the characters in the right place, but it starts with a huge source of psionic energy being broadcast across space. The X-Men picked this up in their own Marvel Comics dimension and chased it down. The Enterprise of the Star Trek universe stumbled upon it as well, bringing them into conflict with a strange new threat known as the Shi’ar Imperial Guard. The source of this energy was the planet Delta Vega, familiar to all fans from the depths of Star Trek lore.
In the first episode of the original Star Trek TV series (after the pilot), the Enterprise encountered an anomaly in space that transferred telekinetic abilities to one of their crew members, Helmsman Gary Mitchell. Eventually the power became too much for Mitchell, and Captain Kirk had to make the decision to abandon him on the planet Delta Vega. But Kirk had no idea of knowing the spatial rift near this planet was also a doorway to the X-Men universe. Unfortunately, it turns out the mutant Proteus found the rift during his cosmic exile, and inhabited Mitchell’s body.
By making that connection this issue brings the two teams together, and the X-Men/Star Trek crossover makes actual sense. Both crews are fighting for the best of humankind -- one from a place of authority, the other a place of stigma, but the issue takes time to note how each was impressed with the existence of the other. There are also great moments delivered as the teams get to know each other (with both Beast and Bones responding to a call for “Dr. McCoy” a high point, beaten only by Spock dropping Wolverine with a Vulcan nerve pinch). Since it’s Star Trek, there’s even a message about guilt, friendship, and how to stop the corrosive abilities of power. After all, Gary Mitchell was a crewman before he was driven over the edge, and the same goes for Proteus, son of Moira MacTaggert.
Pulling all of these themes together while keeping it fun and lighthearted, the two least likely properties to fit together actually tell a story no fan of X-Men or Star Trek should miss.