After defeating Separatist forces on a rain-soaked planet, High Jedi General Obi-Wan Kenobi and Jedi General Anakin Skywalker were dispatched to the Outer Rim world of Nelvaan after Republic Intelligence reported that General Grievous made frequent trips to, and was currently, there. They departed immediately and arrived over Nelvaan with several Venator-class Star Destroyers.
After interrupting a Nelvaanian coming of age ritual hunt, Skywalker and Kenobi were taken to their village, Rokrul. There they learned that all the Nelvaanian males in the village had embarked on similar quests and never returned. After enduring a Nelvaanian ceremony, Skywalker embarked on a quest to discover why Nelvaan was shrouded in seemingly eternal winter and to find the missing Nelvaanian warriors. This was the final Jedi Trial Skywalker had yet to face.
Following the Force, Anakin entered a cave that led deep beneath the surface. There, the Force revealed to Anakin in a vision that he would become Darth Vader. Then, he awoke and discovered a siphon generator that was powered by a captured geothermal crystal. This machine kept the Rokrul region in its wintry state by draining all the thermal energy from the land.
He also saw that there was a base above him and climbed up to investigate. There, he discovered B1 battle droids on guard duty, proving that there was indeed a Separatist presence on Nelvaan. Continuing his investigation, he discovered a pair of Skakoan combat engineers in their pressure suits conversing with each other. From their conversation, he learned that the Techno Union was creating mutant cyborgs from Nelvaanian captives for General Grievous and that there was only one unaltered Nelvaanian captive left—Harvos.
Turning around, a horrified Anakin discovered an overweight Nelvaanian floating in a glass tank immersed in a green liquid. The unfortunate Nelvaanian had been chemically enlarged and one of his arms had been replaced with a blaster arm. Continuing, he saw more Nelvaanian captives. At the end of the chamber, he discovered the unaltered Nelvaanian Harvos in a glass case being gradually filled up with green liquid. Surrounding the cylinder, was a squad of B-1 battle droids. Above the chamber were a few Skakoan combat engineers. One of them pressed a button on a remote that activated the weighting process of Harvos. Slowly, the transformation began.
An enraged Skywalker then intervened and disposed of the squad, though the transformation still continued. The Skakoans then unleashed the mutant Nelvaanians. They were controlled by control harnesses, though Anakin did not realize their purpose. He tried to plead with the mutant Nelvaanians saying that they had been altered and had to regain control of themselves. However, they attacked him with their blaster arms and the young Jedi Knight was forced to defend himself (though he did it mainly with his Force Pushes, trying not to wound them).
Suddenly, Harvos—who had been altered—managed to burst free of his restraints and smashed the tank. He then ripped the cybernetic chestplates off two of the Nelvaanians. Anakin soon got the idea and cut the rest off using his lightsaber.
After that, Harvos spoke to his brethren, reminding them of their heritage and that they had been altered, and then telling them to attack the Techno Union engineers. After dispatching his fellow Nelvaanians, he spoke to Skywalker and pointed to the siphon generator, telling him that he had to free the trapped crystal in order to end the eternal winter.
The enraged Nelvaanian warriors then rampaged through the facility, tearing battle droids apart with their bare hands, and freeing their comrades from captivity. Because they had been chemically enlarged, they had a significant advantage in the battle. The Skakoan combat engineers at the base fled for the safety of their Hardcell-class Interstellar Transports, which had been parked above the base on the surface.
Meanwhile, Anakin leaped to the top of the siphon generator and then lowered himself into the crystal chamber. He reaches his prosthetic arm into the energy sphere of the generator. He used the Force to pull the energy crystal out of its socket and shattered it in his mechanical hand. The generator exploded after and Anakin had leapt to safety as his hand self-destructed. In a result, the weather began to change dramatically and the snow began to melt.
The Nelvaanian warriors reached the planet's surface, where the waiting battle droids were destroyed as the planet's artificial ice age came to an end.
An angry Skywalker clambered to the surface, where he viciously created a catastrophe which resulted in all 5 of the Skakoan engineers getting hurt or killed while trying to flee in their ship. First, he shattered a ramp with Skakoans on it with the Force and then threw more down to the Nelvaanians who finished them off. However, one of the Skakoans survived and tried to shoot Anakin. The Jedi heard the click of his blaster and used Force choke on him, damaging his pressure suit and causing his body to decompress.
Seeing Anakin's destructed arm, the Nelvaan overweights worshiped him as "Holt Kazed"—"ghost hand"—their savior, and in solidarity they tore the implanted weapons from their own arms. Anakin and the Nelvaans returned to the village shortly after the victory. The females villagers were at first silent by the mutants, but when Harvos's child recognized her father, the familial bonds were restored. There were celebrations in the Nelvaanian village. After that, Obi-Wan Kenobi and Anakin Skywalker returned to their Venator-class Star Destroyer.