An unseen older woman tells the story of her father, Elmer Elevator (Jacob Tremblay), when he was a kid. He and his mother Dela (Golshifteh Farahani) owned a candy shop in a small town, but were soon forced to close down and move away when the people of the town moved away. They move to a faraway city where they plan to open a new shop, but they eventually lose all the money they save up while getting by. Dela struggles to find work in the city and is barely able to afford her weekly rent payments to Mrs. McClaren (Rita Moreno).Elmer soon befriends a cat (Whoopi Goldberg) and eventually gets the idea to panhandle the money needed for the store, only for his mother to tell him that it is a lost cause. Angered, Elmer runs to the docks to be alone. The Cat comes to him and begins speaking to him, much to his shock. She tells him that on an island, Wild Island, beyond the city lies a dragon that can probably help him. Elmer takes the task and is transported to the island thanks to a bubbly whale named Soda (Judy Greer). Once they make it to Wild Island, Soda explains that a gorilla named Saiwa (Ian McShane) is using the dragon to keep the island from sinking, but it remains ineffective. The Island continues to sink and needs more and more frequent effort from Boris to pull it back up from the ocean.Elmer frees the dragon, a goof ball named Boris (Gaten Matarazzo), and they go on an adventure in search of a tortoise named Aratuah to find out how Boris can keep the island from sinking for the next century. Boris explains that his kind has been saving the island as long as anyone can remember, and after he succeeds, he will be an "After Dragon". Dragons have been coming to wild island forever as a rite of passage. when Dragons are 10 yrs old (100 in human yrs), they come to the island and save it from sinking (coz the island sinks every 100 yrs), and this turns them into a After Dragon, more muscular, fire breathing and basically awesome. Boris knows that the way he is saving the Island is not the right way, as he hasn't turned into the After Dragon yet. Boris has no idea how to save the Island the right way, and believes that Aratuah would know since he was around when the last dragon Horatio came to save it.During this conversation, it is discovered that Boris cannot fly due to breaking his wing when Elmer saved him (They were chased by Saiwa's 2nd in command Kwan who threw a fire torch at them and ended up hitting Boris's wing, injuring him), and he reveals that he is afraid of both water and fire. The two make an agreement that Boris will help Elmer raise enough cash to buy a new store and will let the dragon go free once finished. Along the way, they encounter some of the islands inhabitants, like Cornelius the crocodile (Alan Cumming), the tiger siblings Sasha (Leighton Meester) and George (Spence Moore II) (The tigers want to eat Elmer, as he is soft and sweet. He only escapes by making them fight over a cinnamon flavored bubble gum) and a mother rhino named Iris (Dianne Wiest) (she fell into a trap with her baby rhino and wasn't able to climb out. Elmer fell into the same trap) all while trying to evade Saiwa and his monkey army. Elmer explains to Iris that he is taking Boris to Aratuah to find out how to save the Island for real. When Saiwa arrives, Iris helps hide Boris and Elmer and tells Saiwa that they went towards the summit if the Island. As Boris and Elmer make their way to Aratuah, the island continues to sink gradually. Kwan almost captures Boris, but Saiwa makes him let go, as he wants Kwan to save the monkeys from the incoming sea. Saiwa tells Kwan that one day he will lead the monkeys, and he has to learn to not abandon them, ever. They still have time to capture the dragon and save the island.They soon make it to Aratuah's shell, but Elmer finds out that he died, and leave as the island continues to sink. While resting on a flower, they are found by Saiwa and his forces. When Boris tries to escapes, Saiwa captures him by his injured wing, fixing it inadvertently in the process. Boris flies away with Elmer. Saiwa reveals that he knew about Aratuah's death, which angers his macaque 2nd-in-command Kwan (Chris O'Dowd), who proceeds to use a giant mushroom as a raft to leave the island, convinced that it is hopeless to save. While flying with Boris, Elmer has an epiphany; it is the roots below the island that pull it down. He manages to convince Boris to fly with all his might and while Boris manages to free the island from two of the roots, he then realizes that this is still the wrong way to save the island. He knows he has to jump into the summit fire to save it. But Elmer is not convinced and keeps telling him not to. Boris pushes Elmer off, who falls off the Island and into the water.Elmer is saved by Saiwa, who reveals that he and the other animals are evacuating the island and berates Elmer for wanting to use Boris for his own merits. He then reveals that once he found out about Aratuah's death, he was frightened about making everyone more worried and was even more worried when Boris showed up to save the island but Saiwa was weary of his goofy personality, so he lied and said he knew how to control Boris. Wanting to fix things, Elmer goes back to the island where Boris tells him he has found a way to save it by jumping into the fire at the summit of the Island and eventually after a lot of encouragement from Elmer he jumps in, he bursts out and magically lifts the island from the sea, finally becoming an After Dragon.After telling the animals of the island how to always let future dragons know the right way to save it, Boris takes Elmer home, passing by a surprised Kwan residing over on tangerine trees. Elmer reunites with his mother and the film ends with him embracing his new life in the city with his daughter narrating the end of the story.