While searching for the Toru Diamond, the Rangers encounter an alien named Tyzonn. Tyzonn came to Earth from the planet Mercuria to stop the evil Fearcats who are intent on releasing their army which had been previously imprisoned in a mirror world. After the loss of his fiance Vella's apparent demise, Tyzonn is reluctant, but is eventually convinced to join Operation Overdrive as the Mercury Ranger. Together the Rangers encounter Thor and Loki, two of the greek gods. Their involvement leads to the discovery of the Blue Saphire, which is stolen by Kamdor and Miratrix.
When Thrax, son of Lord Zedd and Rita Repulsa escapes imprisonment from the Sentinel Knight, he recruits Flurious, Moltor, Kamdor, Miratrix, and the Fearcats to form an evil alliance and destroy the Rangers' connection to the Universal Morphing Grid. The Rangers must go on a quest to seek out the legendary sword Excelsior which is capable of restoring their powers and the Sentinel Knight. With the help of some former Power Rangers - Adam; the Black Power Ranger, Xander; the Green Mystic Ranger, Bridge; the Red S.P.D. Ranger, Kira; the Yellow Dino Ranger, and Tori; the Blue Wind Ranger, the Power Rangers unite and defeat Thrax which fizzles the evil alliance.
After the Rangers encounter a virus, Mack makes a startling discovery about himself. Realizing that he is an android, Mack begins to question his very existence, but he still pulls through for his friends by merging with the Sentinel Knight to become the Red Sentinel Ranger. Meanwhile, when another Fearcat named Crazar shows up, Tyzonn discovers that Vella is still alive. Also the Rangers uncover the Star of Isis, the fourth jewel of the Corona Aurora.
The Octavian Chalice holds the power to uncover the final jewel to the crown. In an ultimate struggle for it, the Fearcats are destroyed for good and so is Kamdor, but not before imprisoning Miratrix inside a gem for eternity. Flurious destroys Moltor and steals the crown and gets his hands on the jewels as well. Now with the power of the Corona Aurora at his disposal, Flurious freezes all of San Angelas. In a desperate attempt to save the world, Mack sacrifices himself to destroy Flurious once and for all. The Sentinel Knight appears and uses the Corona Aurora to bring Mack back to life. As Mack awakens, he realizes that thanks to the crown, he is now human. Thanks to Norg, Tyzonn is reunited with Vella and the Power Rangers move on with their lives now that they have saved the world from evil.