Equinox

Voyager encounters a distress call from another Starfleet ship that is stranded in the Delta Quadrant, the Equinox. The Equinox is under attack from an alien species that seem to come in from a parallel universe (shades of Species 8472 here) and attack the ship. Voyager and Equinox join forces and everyone on Voyager is glad to find friendly faces from back home. What the crew do not know is that their new friends are harbouring a secret that betrays the very principles that Starfleet holds dear and that they will go to great lengths to keep that secret.