Star Trek: Voyager - Season 5 Episode 4 -

The crew of Voyager discover a simulation of Starfleet Headquarters being run by Species 8472, the enemy they helped the Borg defeat in the two-part episode "Scorpion". It is being used to practice infiltrating the Federation. With the aid of regular doses of drugs, Species 8472 are able to shape-shift into human form. It is up to Janeway to try to convince the aliens that Starfleet is not a threat to them.