“One fateful day, Master Vile used the Orb of Doom to reverse the rotation of Earth and turn back time, reverting the Rangers into powerless children (although they retained their memories). While the Alien Rangers and a restored Billy defended the Earth, the children went to different points in time to retrieve a piece of the Zeo Crystal, which was shattered and scattered throughout time by the Rangers themselves. Upon their return, the Zeo Crystal was reassembled, and its power was used to restore the planet back to normal.Their victory did not last long. Rito and Goldar infiltrated the Command Center and stole the Crystal, just before the explosive device they had planted earlier went off, leaving the Command Center in ruins, and the Rangers without a headquarters. While searching through the wreckage, the Rangers found the Zeo Crystal, apparently dropped by the two villains, and then fell into the underground Power Chamber, their new base of operations, where Zordon and Alpha-5 retreated to during the explosion.
While celebrating the defeat of the Power Rangers, Zedd’s palace is attacked by the Machine Empire, who wants to take over the Earth as well, causing Zedd, Rita, and the rest to evacuate and head for the M-51 galaxy to live with Master Vile. Using the power of the Zeo Crystal, Tommy, Kat, Rocky, Tanya and Adam became the Zeo Rangers, Earth’s only hope against the Machine Empire.
Mondo’s plan to divide and conquer was working, and for the Rangers to succeed, they needed help from the Gold Ranger, Trey, who travels to planets where evil needs to be defeated. With the Golden Power in danger of being lost, they were given to former Red Ranger Jason, who joined the team as the sixth Zeo Ranger until it was time to return the powers to Trey. Shortly after Trey reclaimed his powers, Rita and Zedd returned and blew up the Machine Empire’s leaders, ending the threat they posed to Earth.
However, the void left by the destruction of the Royal House of Gadgetry along with Rita and Zedd taking a vacation lead to a new villain’s arrival in the next season.”
- Taken from: http://powerrangers.wikia.com/wiki/Zeo