Power Rangers: Zeo is the fourth season of the Power Rangers franchise that is a continuation of the television series Mighty Morphin Power Rangers, that aired in 1996. It is based on the Super Sentai series Chouriki Sentai Ohranger. Power Rangers: Zeo is the first series in Power Rangers that is part of the annual Ranger suit change to match the annual change of the Super Sentai series.
In the series, after recovering from the destruction of the Command Center, the Rangers discovered the Zeo Crystal intact in the rubble. The Zeo Crystal guided them to a portal, which took them deep underground, where Zordon and Alpha 5 had survived by hiding in the heretofore unmentioned Power Chamber.
At this time, a new set of villains called the Machine Empire decided to take control of the Earth. They chased Rita Repulsa and Lord Zedd away from the moon, and Rito and Goldar were left on Earth, with no memories of their past.
The Rangers become the Zeo Rangers, empowered by the Zeo Crystal. Tanya Sloan joined the team as Zeo Ranger II - Yellow, while Billy Cranston became their technical advisor. Through their battles with the Machine Empire, they were sometimes aided by the mysterious Gold Ranger.
It turned out the Gold Zeo Ranger was an alien, Trey of Triforia. He was injured in battle, and was forced to pass on his powers to a worthy warrior while he healed. He first tried Billy, but it turned out that Billy had unintentionally gained negative proton energy from the destruction of the Command Center. The powers then went to Jason Lee Scott, the Red Power Ranger from the first generation. Jason was strong and had sufficient energy to hold the powers.
Billy later left Earth to be with the Aquitian Cestria, on Aquitar. Jason, whose life force was declining because of the Gold Zeo Ranger powers, had to return the Gold Zeo Ranger powers to Trey of Triforia in order to save the powers (and his life). Rita and Zedd destroyed the Machine Empire royalty, only to decide to take a vacation, leaving the way clear for the next villain, Divatox