The episode was Season 5 Episode 14 "Once You Go Black, You Never Go Back".
In the season finale, Zack absorbs Rita's evil power crystal and turns on the Power Rangers and Angel Grove. Zordon informs the Power Rangers of a secret cave containing the reversal crystal, but Rita and her Octopig monster have other plans. Can the Power Rangers morph in time to save Zack and Angel Grove?
In the season finale, Zack absorbs Rita's evil power crystal and turns on the Power Rangers and Angel Grove. Zordon informs the Power Rangers of a secret cave containing the reversal crystal, but Rita and her Octopig monster have other plans. Can the Power Rangers morph in time to save Zack and Angel Grove?