A group of demons is released after thousands of years in captivity. They wreak havoc on the city of Mariner Bay, which was once their home, in an attempt to reclaim it. The city's only hope lies in five young heroes chosen by a defense organization known as Lightspeed, who have been given special powers and weapons to defend Mariner Bay - Dana, Joel, Chad, Carter, and Kelsey. Together, they easily thwart demon leader Diabolico's first assault.
When Carter fails to make the right decisions in both training and real battle, he questions his abilities as a Ranger. He must regain his confidence to stop Vypra from destroying Mariner Bay with her tremor inducing spikes.
Diabolico unleashes Whirlin, the tornado monster, the destroy Mariner Bay. The Rangers rescue a shuttle caught in the tornado's path from crash landing, and destroy the monster with the Lightspeed Megazord.
Dana questions her father's faith in her abilities as a Ranger when he sends her on a seemingly unimportant assignment as the other Rangers are sent to guard a fuel cell. When one of Diabolico's monsters turns up the heat in Mariner Bay, she discovers that the Rangers were guarding a decoy, and the real fuel cell's safety lies in her hands. She escapes Vypra and gets the fuel cell to the Aquabase, in just enough time to help her friends destroy the monster.
The arrival of Queen Bansheera prompts an attack on a volcano, which threatens to erupt and destroy Mariner Bay. Dana deals with a hostage situation on a bus before joining the Rangers in dousing the Trifire monster's flames and saving the city from disaster.
Queen Bansheera resurrects two monsters from the past and gives them great power. When the Lightspeed Megazord is not enough, the Rangers must summon Ms. Fairweather's newest creation - the Supertrain Megazord.
Lightspeed's experimental Titanium Morpher is stolen, and Diabolico unleashes the evil Titanium Ranger. The Rangers are able to defeat him with their new V-Lancers, but are unable to destroy him, because he is revealed to be Captain Mitchell's son.
The Titanium Ranger goes on another rampage through Mariner Bay, and Dana tries to single handedly stop her brother. When Captain Mitchell reveals the truth to Ryan - that he had no choice but to let Diabolico save him as they hung off the cliff - Ryan thinks twice about being evil, and gives up his morpher.
Diabolico unleashes his three most powerful monsters who threaten the city with explosive feathers. The Rangers are no match against them, until Ryan comes to terms with his evil past and becomes the Titanium Ranger again to fight for good.
Demonite and his monsters attack the city once more, this time with certain advantages - Demonite creates a powerful clone, and Ryan is weakened by the cobra tattoo given to him by Diabolico. Carter's risky move of firing point blank at Demonite with both the Unilaser and V-Lancer destroys him and the clone.
The two remaining monsters from Diabolico's trio - Thunderon and Falkar, launch a final assault on the Rangers, who are short one Titanium Ranger. When both increase in size and take down the Supertrain Megazord, Ryan risks his life and helps them out in the new Max Solarzord, destroying both monsters.
Diabolico brings back his three generals merged as one monster, Trika, which the Rangers destroy with their new Battle Boosters. Diabolico then goes down himself and battles the Rangers, taking down the Supertrain Megazord. Ryan finds the cobra tattoo's power source, a cobra monster from the tomb, and destroys it, lifting the curse. He joins the Rangers and helps them defeat Diabolico with a new Zord combination - the Lightspeed Solarzord.
Impus completes his transformation into Olympius, who has captured four Rangers, and infiltrates the Aquabase by posing as them. His plot is uncovered by Carter, who weakens him with water, and frees his friends.
When a monster sets a power plant ablaze, Captain Mitchell jumps into action and braves the fire to rescue a little girl. The incident reveals to Carter that it was Captain Mitchell who has saved him from a fire many years back when he was a child.
The demons perform a ritual to return Queen Bansheera to her bodily form. The Rangers interrupt the ceremony, but only enough to bring back the Queen in a deformed state, for which Olympius swears vengeance.
A monster named Cyclopter trains with Chad's old sensei, who resents him for becoming a Ranger and not following the true path of a martial artist. The monster uses his training to battle the Rangers, but is defeated by Chad, who makes peace with his former mentor, and recalls his own training.
The Rangers take the Max Solarzord to space to prevent spores which have attached to them from going off and releasing toxins into the atmosphere. Meanwhile, Loki and Vypra resurrect Diabolico after they are betrayed by Olympius.
Freezer captures four Rangers, with only Carter left to face him and Olympius. Olympius has his own problems, as Diabolico returns looking for payback. Bansheera keeps Diabolico in check, and sends her son to get Carter, who defeats Freezer frees his friends with the new Thermo Blaster.
Joel becomes jealous when he mistakes Clark, Ms. Fairweather's brother, for her boyfriend. She brought him to the Aquabase to help design the new Mega Battle Armor, which Joel and Chad use to defeat Infinitor.
Jinxer loses an egg containing a powerful monster to a quick witted thief who the Rangers are also after. His plot to hold the egg for ransom backfires when the villains attack him, but he is saved by Kelsey, who discovers the thief to have a heart of gold.
Vypra uses Chad's new love interest, a mermaid named Marina, to lure Chad into a trap. When Marina's life is endangered, Chad summons up the strength and courage to defeat Vypra and Loki and rescue her. The two reunite, but only to part ways forever.
Trakeena returns to destroy Earth as vengeance for her defeat at the hands of the Lost Galaxy Rangers, but first, she must absorb enough human life force for power. The Lost Galaxy Rangers travel to Earth to team up with the Lightspeed Rescue Rangers to stop her. Olympius and his demons also enter the fray, sabotaging Trakeena's power drain and mutating her into a giant, maddened creature. The two Ranger teams destroy Trakeena with a Lights of Orion powered Omega Megazord.
A sinister monster gives four of the Rangers amnesia. Dana holds the monster off long enough for Ms. Fairweather to restore the Rangers' memories. When they rejoin the battle, they vanquish the demon with the Thermo Blasters and the Lightspeed Solarzord.
The Rangers are transported into the monster underworld, where they face invincible dead monsters. The escape with the help of the Sorcerer of the Sands, who Ryan has found in the desert in hopes of discovering a way to imprison the demons. Olympius remains trapped in the underworld, and Diabolico kills the sorcerer in a battle of spells.
Jinxer manipulates Captain Mitchell into providing Olympius with enough energy to destroy the Rangers after he escapes the underworld. When Ms. Fairweather interrupts the power feed, the star power is destroyed, allowing the Rangers to defeat the demon prince.
Marina returns to ask for Chad's help after her father's trident is stolen, which a water demon uses to drain the water from Mariner Bay. Chad recovers the trident, as well as a captive Marina, and safely returns the two to Neptune, the god of the sea.
With four Rangers trapped, Carter has a decisive showdown in the Skull Cavern with Bansheera, who has already killed Vypra and Loki in her quest for power. When he strikes her with a battle booster, Carter and his teammates escape the Skull Cavern dimension.
Olympius captures and enslaves Diabolico, and uses him to battle the Rangers and destroy their Megazords. They are just barely bailed out by the new Lifeforce Megazord, which feeds off the Rangers' life energy to destroy both demons.
Queen Bansheera begins a ceremony which will release all of the demons from the shadow world to Earth. Her forces take control of the Omega Megazord and the Lifeforce Megazord. The Rangers escape a raid on the Aquabase, destroy both Megazords, storm the Skull Cavern, and defeat Queen Bansheera once and for all.