Once a Ranger

Once a Ranger is a special team-up episode in Operation Overdrive involving Power Rangers from five different generations (collectively known as the Retro or Veteran Rangers) assisting the Overdrive Rangers in defeating a new threat: Thrax, the son of Lord Zedd and Rita Repulsa. The title of the episode is a reference to the long-standing phrase "Once a Ranger, always a Ranger." The episode also commemorates the 15th Anniversary of the Power Rangers franchise.

Summary
The son of Lord Zedd and Rita Repulsa, Thrax, unites all four Corona Aurora-seeking factions of villains into a new Evil Alliance. When they manage to overwhelm the Overdrive Rangers, their connection to the Morphing Grid is severed, and their powers destroyed! With the gems still needing protecting, Sentinel Knight assembles a team of replacement Rangers, made up of members of previous (or future) teams of Power Rangers.

No longer having Ranger powers, the former Overdrive Rangers return to their civilian lives. All but Mack, who learns of Thrax's plans to destroy Sentinel Knight, and seeks out the only item capable of doing so, the legendary sword Excelsior. Will he be doing this alone, or can the call of action prove the adage of "Once a Ranger, Always a Ranger" to his former teammates? Meanwhile, their replacements, the team of Retro Rangers, head to Angel Grove to seek out the only being capable of repairing the Morphing Grid: Alpha 6! Even if they succeed, can two combined teams of Power Rangers stop Thrax for good, and permanently break up the Evil Alliance?

Part 1
Thrax—son of Rita Repulsa and Lord Zedd—is freed, and he recruits all the villains who have been searching for the Jewels of the Corona Aurora—Flurious, Moltor, Kamdor, Miratrix, and the Fearcats Mig and Benglo—to unite with him temporarily. By severing their connection to the Morphing Grid forever, the temporary Evil Alliance destroys the Rangers' powers, and Andrew Hartford cannot fix their destroyed morphers. They use their enhanced skills in an attempt to defeat the villains, but fare no better, so the Sentinel Knight recruits five Rangers — Kira Ford, Xander Bly, Adam Park, Bridge Carson, and Tori Hanson—to replace the Overdrive Rangers until their connection to the Morphing Grid is restored.

Part 2
The villains in Thrax's "Evil Alliance" are angry, because Thrax did not think there would be more Rangers. Meanwhile, Mack Hartford is searching for information on the sword Excelsior—which Thrax says is the only thing capable of eliminating the Sentinel Knight. He tries to retrieve the sword, but is confronted by Thrax, who intends on saving the legacy of his family of Evil. He is unable to claim the sword for himself, so he sends a giant monster named Vulturas into the city to destroy it. The Retro Rangers head him off in the DriveMax Ultrazord and Flash Point Megazord, but are overpowered, and DriveMax Ultrazord loses an arm in the battle.

Meanwhile, Mack convinces the Rangers that they will always be Rangers, powers or not, and they all attempt to free Excelsior from the Warrior-Goddess's hands. The statue comes to life and gives them the sword, sensing the shared friendship and combined conviction in their hearts. Then, Mack heads to the battlefield, where the Retro Rangers are about to be destroyed, and uses the sword Excelsior to destroy Vulturas. Meanwhile, Adam reactivates Alpha 6 and teleports him inside the Morphing Grid, where he fixes the severed connection, restoring the Overdrive Rangers' powers. Mack hands Sentinel Knight the Excelsior sword, which gives the Knight a Cyborg form.

Confronting the "Evil Alliance", all 11 Rangers morph and battle the villains. Adam battles Thrax. Bridge and Mack team up to defeat Flurious. Dax, Will and Xander defeat the Fear Cats. Tori and Rose battle Moltor and his Lava Lizards, and Kira and Ronny team up against Kamdor and Miratrix. Tyzonn is able to destroy the newly revived Vulturas and Flurious's Chillers. To finish the battle, the Sentinel Knight arrives in his Cyborg form and destroys Thrax, and then joins the Rangers in defeating the rest of the villains.

Trivia

 * Rita Repulsa, Thrax's mother, became the Mystic Mother in Power Rangers Mystic Force, though Xander did not relate the two together.
 * It is unknown when Thrax was born. It could be possible, out of many other theories, that he was born prior to the desruction of the United Alliance of Evil by Zordon's Energy Wave.
 * It is unknown how Alpha 6 ended up on Earth, as Alpha 6 was last known to be on Mirinoi.
 * Angel Grove makes an appearance, its first since Lost Galaxy's Quasar Quest.
 * Adam has a new Morphing sequence, as does Bridge. For Adam's Morph, Adam doesn't yell "Mastodon." As for Bridge's, it doesn't end with Jack's battle cry ("S.P.D. Red Ranger!") or the pose, instead his stance is more like Sky's when he was morphing into the Red Ranger. Bridge's morph is similar to the Cat Ranger morph, likely due to both (Cat and Bridge's Red) only being used for one episode each and not worth doing the more elaborate morphs of the other SPD Rangers.
 * During Mack and Bridge's battle with Flurious, Bridge tosses his Delta Blasters to Mack. In GoGo Sentai Boukenger vs. Super Sentai, AkaRed, being the embodiment of the Red Sentai Spirit, grants the use of the Delta Blaster's counterpart, the D-Magnums along with weapons of other Sentai Red Rangers, to BoukenRed.
 * Adam also seems to have the ability to use the Defender Vest, although it may've been a modification by Andrew.
 * During his battle with Thrax, Adam yells out,"Morphinominal!" a phrase coined by Kimberly Ann Hart in the original Mighty Morphin' Power Rangers.
 * Due to copyright complications, the "Go Go Power Rangers" theme song was not featured during Adam's morphing or fighting sequences. In its place was an unnamed soundtrack that used high-pitched instruments. However, at the end of the second episode, all eleven Rangers put their hands together and called out, "Go Go Power Rangers!" They were able to use the song, however, for Power Rangers Samurai, presumably because of Saban reacquisition of the series.
 * It's unclear how Xander knows anything about piloting actual Zords, as the members of Mystic Force themselves transformed into their Zords.
 * Thrax's "Evil Alliance" could be a reference towards Dark Specter's United Alliance of Evil (an alliance his parents were apart of).
 * This is the first time in the history of the series where another team of rangers pilots the current team's zords.
 * Adam, Bridge and Xander were were all Green Rangers at one point in time. Xander, however, is the only one who still used his Green Ranger powers.
 * Once A Ranger is the fourth teamup in Power Rangers history to feature a combined team of 11 Rangers, proceeded by Power Rangers Zeo/Mighty Morphin Alien Rangers, Power Rangers Lost Galaxy/Power Rangers In Space, and Power Rangers Dino Thunder/Power Rangers Ninja Storm. It is also the second time that Veteran Rangers from more than 2 seasons team up, the first one being Forever Red, which also commemorates the tenth anniversary of the Power Rangers Franchise.
 * The color choices for the team may be a reference to Voltron, including having the black themed ranger as the leader.
 * Right as the rollcall starts, usually it's the previous Power Ranger team who goes first then the newest Ranger team does thier rollcall, instead the newest team rollcalled and the Retro Rangers went second. In the rollcall, however, they all just grunt as they pose.
 * Aside from the Overdrive Rangers and Sentinel Knight, Tori is the only Ranger whose Japanese counterpart, Nanami/HurricaneBlue, was in the Japanese counterpart of this team-up, GoGo Sentai Boukenger vs. Super Sentai.
 * Moltor, Kamdor, Miratrix, and the Fearcats are the only characters with either direct or semi-direct Boukenger counterparts whose counterparts were not part of GoGo Sentai Boukenger vs. Super Sentai. Only Flurious' counterpart, Arch Priest Gajah, was included in Boukenger's team-up.
 * Adam is the only Ranger where no one from his Japanese counterpart's team (Kyoryu Sentai Zyuranger or Gosei Sentai Dairanger, depending on one's point of view) appeared in Gogo Sentai Boukenger vs. Super Sentai (Adam had MammothRanger's suit, but used ShishiRanger's mecha, Mythical Chi Beast Star-Shishi).
 * Both this teamup and the Forever red Teamup involved having some type of weapon or Villian having to with Rita Repulsa and Lord Zedd.
 * The Setinal Knight said that he restored the powers of the previous Rangers however Zander's power's were never destroyed and niether were Bridges Powers also Bridge is not from this time period.