10 Best Star Trek Crossover Episodes
9. Sarek
An astonishingly overlooked episode of The Next Generation, Sarek isn't the most glaring of crossovers but nonetheless a fantastic piece of interwoven storytelling. In fact, in terms of laying the groundwork for Season 5's Unification two-parter, it's arguably one of the most important pieces of shared universe storytelling in the entire franchise.
Its wider implications to one side though, The Enterprise is dispatched to mediate diplomatic talks between the Federation and the Legarans. This is a mission of critical importance, so Ambassador Sarek (yep, Spock's old man) has been dusted off and sent to lead the delegation. However, slightly problem, he's developed a degenerative Vulcan condition that is causing him to a) lose control of his emotions and b) transit all the really violent ones to any nearby humanoids.
Wesley and Geordi nearly come to blows over how they both get their hole, Beverly slaps the taste out of son's mouth, and Riker and Worf walk into Ten Forward to find it the site of a mass brawl. Eventually Picard agrees to Mind Meld with Sarek in order to transfer his emotional stability, but the result is that he himself becomes overwhelmed.
The episode's final act has him pouring the entire anguish of Sarek's soul all over his Ready Room couch, and it's one of Patrick Stewart's most astonsihing performances in the whole series.