This series features the characters from the two mini-series about aliens coming to Earth claiming to be friends but in reality have an ulterior motive. The aliens may have been beaten in the mini-series but there are still a lot of them out there. In this series, Nathan Bates, the industrialist who manufactures the biological weapon that was used to defeat the aliens, wants to get his hands on the aliens' technology, so that he could profit from it, makes a deal with Diana, the alien leader. Rebel leaders, Mike Donovan, Ham Tyler, and Julie Parish know that Diana can't be trusted, so they are constantly trying to figure out what Diana is up to and stop her.

In the first two mini-series, the Visitors spoke with an electronically modulated voice. The sound processing, however, was dropped for the ongoing series. This created a continuity problem with the two mini-series as the Visitors were able to infiltrate human strongholds and vice-versa without their different voices giving them away.

Lydia:
You know, I've never lost in mortal combat.


Diana:
Idiot. If you had, you'd be dead.