Star Trek: First Contact (1996, directed by Jonathan Frakes) finds the Next Generation crew coming into their own. Unburdened by establishing a linkage with the previous series (and with accomodating William Shatner's ego), it has more room to breathe, establish the narratives for its characters, and generally tell an unforced narrative. It's one of the best of the Star Trek films; it's the Next Gen crew's Wrath of Khan. You cold make a case for First Contact as the best of the Star Trek movies. While the previous films have always had a level of rollicking adventure, this one tackles more existential themes. I like that it doesn't have an allegorical ax to grind, which has often been the Achilles Heel of Star Trek. Instead, it tackles science fiction qua science fiction, as a crucible for examining the human heart in conflict with itself rather than as a sociological funhouse mirror.
The story finds the Borg invading human space at last. The Enterprise had already encountered the Borg, a race of cyborgs who destroy whole races in order to absorb their technology and biology into their own collective, six years earlier. In that encounter, Captain Picard was captured by the Borg and assimilated into them. Star Fleet, understandably, doesn't trust Picard and relegates the Enterprise to patrolling the Romulan Neutral Zone while it does battle with the Borg cube ship. Picard, sensing that the battle is going badly, ignores his orders and joins the battle anyway. His knowledge of the Borg turns the tide, but they escape into a time warp. The Enterprise follows. The Borg's intent is to travel back in time to prevent the inventor of warp technology, Zefram Cochrane, from making his maiden flight, which is the lynchpin to the founding of the Federation. The Enterprise's dual task is to make sure that history is maintained, and to fight off a Borg invasion of their own ship...