The Borg are the best enemy devised against the Federation in the Star Trek universe. In Dark Frontier, a made-for-TV "film", we get to see how Seven of Nine (Jeri Ryan) from Star Trek: Voyager was assimilated by the Borg before she was adopted by Captain Janeway (Kate Mulgrew), and how she handles being re-united with the Borg collective.
The plot begins as Janeway and the crew decide to steal a transwarp coil from a crippled Borg sphere. With the Borg technology, the Voyager crew could shave years off their journey home. (For those of you not in know, Voyager is stranded in the Delta quadrant, which is a 70,000 light years away from earth.) As the Voyager crew get their hands on the transwarp coil, they lose Seven of Nine on the away mission who expresses her desire to re-join the Borg. A great conspiracy involving Seven's place on Voyager and her role in the Borg's second attempt at conquering earth is then revealed to us by the Borg Queen (Susanna Thompson). The outcome is predictable: Jeri Ryan is needed for the audience ratings; the Voyager crew use the single transwarp coil they stole to reduce their voyage by fifteen years.
Being nit picky for a moment, the Borg Queen says that Seven of Nine is the first Borg to become an individual. However, the writers appear to have forgotten Hugh who appeared a couple of times in Star Trek: The Next Generation episodes.
Dark Frontier is a classic Star Trek episode, containing the right mixture of suspense, action, effects, and philosophical introspection---I highly recommend it. The only thing that bugs me is that between this and a few other episodes where Voyager have managed to take "shortcuts" home, I figure they've should've been reached the end of their journey by now! Is anyone keeping track? [According to several people, all the short cuts still leave a 20+ year journey.]