Mia and Mario are working at the farm, when Franca stops by on her way to Contessa's place. Tarzan the dog manages to sneak into Franca's scooter before she leaves, and Mia and Mario must hurry after her. Meanwhile, Violetta is in a gazebo, looking at books with illustrations of unicorns, before looking happily at the necklace with the copy of the blue gem on it. She puts it on as Contessa calls out for her to get back to training. Mario tells Mia to wait for him outside the property, while he goes to look for Tarzan. Spotting Mario, Violetta goes to talk to him and asks him to help her with one of her riding boots. While waiting, Mia spots Tarzan, and tries to get Mario's attention. While Mario keeps Violetta's attention, Mia sneaks past him and into the stables where Tarzan is.
Mia's bracelet begins to glow, and so does Violetta's necklace. Violetta quickly hurries away from Mario, realizing what is happening. Mia finds a quiet spot to read the new riddle, and Violetta returns to the gazebo, seeming excited about going back to Centopia.
Yuko and Mo are helping Lasita with looking after the animals as Mia arrives. Mia asks about Varia, with Yuko commenting she hasn't seen her since last time Mia was there. Violetta then comes crashing down, Mia commenting that she still seems to be working on her landings. Mia still seems to feel suspicious about "Varia".
Rixel reports to his master, who seems to be losing his patience with him. Rixel has to get to the Rainbow Isle before the elves do, leaving Gargona to look after Tuquito. Mia tells the group about the latest riddle, and wonder if Xolana and Shiva have found the Jade Horn yet. Violetta wants to join the group on their quest, but Mia tells her it might be dangerous. Since Violetta has trouble flying, Mia suggests Varia stay and help Lasita with the animals. Violetta is initially annoyed, but cheers up when Lasita offers to help teach her how to fly when they're done cleaning.
The group find Xolana and Shiva near a spot they believe might be where the Jade Horn is, and by removing some of the plants growing on a nearby rock formation, they find that it is indeed the Jade Horn they were looking for. Shiva and Xolana fly back to the palace to report their findings, while the others try to figure out to reach the Rainbow Island. Onchao is able to activate it with his horn, and the group decide that one of them should stay behind to guard the horn in case something happens. Mia, Yuko and Onchao decide to head out, with Mo staying behind. He gives them a bottle of berry juice, which Phuddle suggested they use to contain the rainbow water.
Back at the crater, Lasita is helping Violetta learn to fly. Xolana and Shiva return and explain what they learned, and Violetta decides to go catch up to Mia and the others, wanting to join them on their quest.
Exploring the Rainbow Island, they meet a Rainbow Unicorn who speaks to them, revealing that all unicorns are capable of speech while on the Rainbow Island, including Onchao. Onchao introduces himself to the daughter of the older Rainbow Unicorn, who tells them more about Ono. Meanwhile, Violetta is slowly making her way to the beach. Rixel spots her as he searches for the elves, deciding to follow her since she might lead him to the others.
Mo asks why Varia is there, when Rixel suddenly leaps forward on Gurga. Having no other choice, Mo and Violetta fly up to the Rainbow Bridge to warn the others, but Mo's frienship ring falls off. Onchao and the younger of the two unicorns from the Rainbow Island seem to be getting along well, as they lead them to the source of the rainbow water.
The elder of the two unicorns of Rainbow Isle explains that only the unicorn king will be able to get the water from the source. Mia believes this must be Onchao, since Ono is not around. However, the source of water won't give him the rainbow water, meaning Onchao is not the current king of the unicorns. As they arrive on the island, Rixel following shortly behind, Mo realizes he has dropped his ring, and thus can't warn Mia and Yuko. He tells Violetta to stay behind while he goes to find them, and Violetta watches in horror as Rixel orders Gurga to start burning the place. She tries to stand up against him, but without any means of defending herself, Rixel is not intimidated. Mo returns with Mia and Yuko, and Rixel threatens to burn Varia. Mia holds up the bottle of berry juice, claiming she has already obtained the rainbow water. Yuko and Mo save Violetta, while Mia leads Rixel away from the island. Mia has Onchao deactivate the bridge while Rixel and Gurga are on it, dropping them into the ocean.
Mia finds Mo's ring, and brings it with her as they return to the island. Mia asks if Varia is the reason Rixel found them, but Mo takes the blame. The group try to figure out who the unicorn king is, if Onchao is not the one. Since Onchao is the only son of Ono, the group come to the conclusion that Ono must still be alive somewhere. The group bid the two rainbow unicorns farewell, and Mia returns Mo's ring. As Mia's bracelet and Violetta's necklace beginning to glow, Violetta says she has to leave and walks over to the bridge. Mia realizes she must head home too, and says good bye to the others.
Mia and Violetta return to their own world, and Mia notices that Tarzan and her book are missing. She comes across Mario, who reveals himself to have found her bag and Tarzan. Mia thanks him, and asks if he looked through the book, but Mario insists he didn't, believing the book to be Mia's diary. Mario asks where Mia disappeared to, and she promises that she will tell him someday as the two head back to the farm.
- Chekhov's Gun: Phuddle gives the elves a bottle they can use to bring the Rainbow Water back with them, except the bottle is already filled with berry juice. Later, Mia uses it to trick Rixel into believing they've obtained the water.
- Suddenly Voiced: Unicorns are able to talk while on Rainbow Island.