On the planet Roon, Mungo Baobab, Auren, Nilz and the droids R2-D2 and C-3PO continue their search for the Roonstones. Before leaving, Auren bids her mother Bola Yomm farewell and promises to return. She also tells Nilz to look after their daughter.
The droids ride a large animal with ten legs called Mogo. C-3PO finds the ride uncomfortable, encountering a bees' nest, an ape and even R2-D2. The travelers later reach the Pinnacles of Felth. Nilz believes that they will have a good look of the Roon comets from this peak. The droids dismount. Using a pair of macrobinoculars, Mungo observes the comets and says that they will complete their orbit of Roon in a few days and head into deep space for another year. Mungo wonders whether the "Cloudcraft" they borrowed from Governor Koong has enough juice to get them back to Manda.
Meanwhile, Governor Koong gives Admiral Terrinald Screed an aerial tour of the province of Umboo, which he describes as a land of wealth and plenty. Screed contemptuously remarks that he is not surprised that the one area worth having on Roon is the one area he cannot take. Screed regards Koong as a weak and unreliable ally. As a show of force to impress the Emperor Palpatine, Koong sprays Umboo with a red biological weapon.
Mungo, the Yomms and droids spot Koong's starship releasing chemical weapons. Artoo's sensors detect germ contamination. R2-D2 advises them to remain her until the cloud passes from the valley. Auren points out that her mother Bola is down there and that they must help here. Nilz says that C-3PO is right and tells his daughter that they must stay here till the poisonous cloud passes.
Aboard his starship, Governor Koong tells Screed that he deployed the Rooze Disease. Screed recalls that the Rooze Disease was a strain of germ warfare used during the Mavvan Conflict. Screed warns that the disease is too deadly, and that when the Galactic Empire tried to use it against their enemies, they killed off many Imperials. He says that they never used it again and chastises Koong for his stupidity. Koong tries to defend himself but the klaxon warns that a leak has been detected in the Rooze containers and that the germs are seeping into the ship's ventilation systems.
Admiral Screed orders the ship's computers to activate the decontamination fields. Governor Koong tries to tell them to shut down all intakes to the command bridge but it is too late. While the Admiral is able to quickly don a protective breathing mask and his stormtroopers engage their armors' air filtration functions, Koong is fully exposed. The Kobok Gaff's species is immune to the Rooze Disease. The ill Koong orders Gaff to bring him Nilz, the only healer on Roon who could possibly find a cure.
Returning to the Yomm homestead, the Yomms, Mungo and the droids find that the trees have withered. They also find that Bola has been afflicted by Rooze Disease. While Nilz and Auren tend to her, C-3PO asks what kind of "creature" would do such a thing. Mungo blames Koong. They are soon confronted by Gaff and a pair of stormtroopers, who have been sent by Koong to capture Nilz. Gaff tells them that Koong has been infected by a leakage of the Rooze Disease.
Auren charges at Gaff with a shovel. Mungo intervenes and knocks down the two stromtroopers. Gaff shoots the shovel out of her hand before knocking Gaff out. Mungo also charges at Gaff but he uses his blaster to hold back Baobab. He orders Nilz at gunpoint to bring his medical bag and to also bring Auren as insurance against Master Mungo trying anything. Mungo discreetly passes half of his memling into Auren's belt while Nilz sprays a potion over the ill Bola in order to slow the Rooze infection. Bola tells him to be careful.
After Gaff and the stormtroopers leave, C-3PO tells Mungo that R2-D2 says that the Rooze will not spread as fast if the victim lies still and calm. Mungo thinks that they will still have a chance to safe Bola and the others. R2-D2 beeps in agreement. C-3PO says that a cure for the disease must be found within three days or the victim will fade away into nothingness.
Later, Mungo and the droids fly their Cloudcraft. C-3PO asks Mungo if he has a plan to rescue Auren. Mungo shows him the memling which his Uncle Oggem Baobab gave him. C-3PO notices that it is smaller. Mungo explains that he planted half of it on Auren before Gaff took her away. He explains that the memling serves as a navigational guide to help find its other half and Auren. R2-D2 also passes them the seed gun and the pyramid. C-3PO apologizes for not being able to translate the messages on the pyramid despite consulting his datatapes. Mungo explains that he knows that the writings provide clues to the Roonstones but they are meaningless if they don't find Nilz and Auren. C-3PO and R2-D2 observe Mungo's feelings for Auren and her people, which Mungo acknowledges.
At Koong's Fortress of Tawntoom. Nilz treats the ill Koong, who is growing impatient and warns Nilz that the life of his daughter Auren depends on his good behavior. Admiral Screed visits the Governor, telling him that he and his men will be leaving in the afternoon. Koong is horrified and reminds Screed that he promised him stormtroopers and a Star Destroyer. Screed responds that this was conditional upon delivery of the Roonstone fortune that he promised the Admiral. Koong claims that Mungo will lead them to the Roonstones. The Admiral however has had enough and storms out of the .
Approaching Koong's fortress, Mungo gets Artoo to circle the Cloudcraft around the main crater of the volcano. They fly through a backdoor which brings them through a tunnel full of lava. Despite escaping the volcanic eruption, their ship crashes in snow on the other side. The trio land on the snow. Following the memling, C-3PO asks R2-D2 if his censors are picking up anything. C-3PO is startled by a mirror but R2-D2 realizes that it is a mirror. Holding the pyramind against the mirror, Threepio realizes that the it is written backwards. The message says that the "Roonstone treasure you desire lies buried above Tawntoom's fire."
Mungo realizes that the Roonstones are hidden within the walls of this volcano and that Koong has been sitting on a fortune that he was unaware of. R2-D2 takes an interest in the mirror. Just then, an avalanche strikes outside. C-3PO and Mungo are caught up in the avalanche but R2-D2 falls into the mirror, which causes him to shrink. Mungo realizes that the mirror is some kind of disintegrator and warns C-3PO not to touch it. Threepio and Mungo soon encounter a shrunken R2-D2, who has been shrunk to the size of a toy. Mungo says that Artoo will have to wait for the mirror's effects to wear off. Artoo panics when C-3PO wonders whether this change is permanent. He stores Artoo inside a chest compartment.
Inside Koong's laboratory, Nilz tells Auren that he is close to completing the cure for the Rooze Disease. Auren is worried about her mother and whether Koong will let them go home once they have delivered the cure. Nilz warns that Koong is not known for his generosity. Auren expresses frustration until she discovers the second half of Mungo's memling in her belt. Nilz reasons that Mungo knew that the two halves would attract each other. Auren realizes that Mungo is coming to rescue her but thinks he has little chance against Koong's men.
Mungo and C-3PO climbs the stairs to Koong's fortress. Mungo also spots his impounded ship, the Caravel. He intends to use the ship to travel to Manda once they have rescued Auren and Nilz. The two infiltrate the hangar bay. Mungo and Threepio use the mini Artoo as a distraction to steal some robes from tentacled alien works in order to disguise themselves. Mungo restrains the aliens using vines from his seed launcher.
They then approached two guards. As Mungo distracts the guards with pies, Threepio sneaks the mini Artoo through a ventilation shaft to find Nilz and Auren. He travels through the ventilator shaft and approaches their laboratory, which is guarded by the sleeping Gaff. Gaff's pet Goorl discovers R2-D2 and attempts to chase him but the droid uses his grappling hook to get inside the laboratory. Artoo finds Nilz and Auren, who realize that he's fallen into one of Koong's molecular disruptor traps. Artoo tells Auren and Nilz that Mungo is here as R2-D2 starts growing back.
Nilz tells Auren and R2-D2 he has developed two batches of cures for Governor Koong and one secretly for Bola and their stricken neighbors. R2-D2 helps them escape the laboratory. Nilz overpowers Gaff with the Rooze cure, which causes him to break out laughing uncontrollably. Back at the hangar, C-3PO and Mungo wait for R2-D2, Auren, and Nilz. The guards demand more pies. When the Yomms arrive, Mungo knocks out the guards. C-3PO notices that R2-D2 has grown back. Auren hugs Mungo and thanks him for finding them.
Meanwhile, Koong attempts to convince Screed to stay by telling him that his aerial drilling platform is able to search out the source of the Roonstones at his command. Screed is skeptical and tells him to focus on looking for a Rooze cure instead of the Roonstones.
Mungo, the Yomms and droids head for the parked Caravel but Gaff warns Koong and Screed that Mungo and his friends are escaping with the Rooze cure. While the Yomms go to start the Caravel, Mungo takes the serum and commandeers the aerial driling platform with the help of C-3PO and R2-D2. Mungo reveals himself and offers a bargain: if the Imperials leave Roon alone, he'll give up the cure to the disease and the source of the Roonstones.
R2-D2 and C-3PO struggle to control the aerial mining platform. After some difficulties, the droids shoot a portion of rock near Koong's fortress which reveals a large trove of Roonstones. Based on this discovery, Screed orders his troops to seize Koong's fortress. Admiral Screed reneges on his bargain with Koong, telling him that the Empire would not form an alliance with a petty tyrant like him. Determined that nobody else but himself acquire the Roonstones, Koong charges up the aerial mining platform and fires at the Roonstones, causing them to fall into the lava. This causes the lava to freeze, destroying the treasure.
R2-D2 and C-3PO tell Master Baobab that the volcano is the only heat source for Koong's fortress. When the lava freezes, so will they. Admiral Screed orders an evacuation. Mungo attempts to board the Caravel but is blocked by Gaff. R2-D2 charges at Gaff, knocking him to the ground. Gaff tries to get to his feet but injures his leg. Mungo and the droids flee aboard their ship. A bitter Koong blames Mungo for destroying his fortress, his treasure and his hold on Roon. He says he has nothing left including his health. Before they escape, Mungo gives Koong the cure, but it is too late as the Rooze disease makes him disintegrate.
Aboard the Caravel, Nilz says that Koong's tyranny is over. Though Mungo was unable to acquire Roonstones, he finds true love in the form of Auren. The two embrace each other. R2-D2 also reveals that he has kept a small Roonstone inside his compartments.
Back at the Yomm homestead, the trees have started to grow back. They return to Bola and cure her off the Rooze disease using the serum. Mungo tells C-3PO that he intends to return to Manda to inform his father that he is safe. He tells the droids that the people of Umboo will need their help to set things right again. He tells the droids that they are the best friends he could ever hope for. C-3PO admits that he and R2-D2 have an affection for their master. Mungo promises Auren that he will return before hugging her. The droids and Yomms farewell Mungo as he departs for Manda on the Caravel.
This episode was released on DVD in 2004 as part of the Treasure of the Hidden Planet feature film.