Jake's father, Jonathan Long, embarrassingly dropped him off at school during a rainy day. Jake began to feel being a 14-year-old had no good effects on his life, wishing that if he were an adult he could drive a car and handle his own things.
Just then, Fu Dog contacted Jake to get to the Magus Bazaar to investigate a reported wild magical creature terrorizing the place. The trio arrived and witnessed the creature. Jake sent a photo to Fu Dog and Lao Shi, and warned them that the creature is an Avemetrus, who is as old as time itself. Fu Dog failed to warn Jake, Trixie, and Spud to stay away from its tentacles as it will suck their youth. Jake attempted to fight the creature, but the Avemetrus grabbed the three. Jake then managed to escape while Trixie and Spud's youths were unknowingly sucked by the creature. Luckily, Jake managed to scare away the creature and it fled.
Jake returned home, heavily tired, and went straight to bed. The next day, Haley walked into Jake's room and noticed an unfamiliar person in Jake's bed and attacked him. However, she was shocked to see that it was actually Jake, and he was now a 21-year-old. Jake tried to escape without being seen by his parents. Haley helped her brother to make his way up to the chimney, but Jake struggled to climb up it. His father then created a fire and burned Jake, shooting him straight out of the chimney. After crashing, Jake got dressed by other people's clothes and stumbled upon a now-elderly Trixie and Spud.
The three of them quickly got to Lao Shi's workshop, where Fu Dog explained that their youth had been stolen by the Avemetrus. When Jake questioned on how to revert them back to their original age, Fu Dog told them that the Avemetrus had to be vanquished. While they investigated the Avemetrus' whereabouts, Trixie and Spud were forced to stay at a magical retirement home for the elderly.
However, Jake, realizing that was matured enough to do anything as an adult, began driving lessons and performed horribly. However, he managed to convince his driving teacher to give him his license, in fear of ever being stuck teaching a horrible driver, just as long as Jake never went near the driving teacher's neighborhood. Jake continued to pursue things as an adult, such as buying a car, watching PG-13 movies, and even purchasing his own a luxury suite by just using credit card, even though he had to pay it all back. Trixie began to feel that Jake wasn't focusing on hunting down the Avemetrus, as he was too caught up of the lifestyle of an adult and did not even care about the struggle that Trixie and Spud were having being elderly. Jake felt sorry for them, so to make it up to them, Jake invited them to his luxury suite for a big party before they returned to their normal age,
Hours later, Jake's suite was ready for the party. However, before it starts, Fu Dog tried to contact him, but Jake ignored his call. Unbeknownst to him, Lao Shi encountered the Avemetrus and couldn't handle to fight it alone. He then decided to fall back, but got ambushed by the Avemetrus' tentacle.
Back at the suite, Trixie and Spud made it to the party. However, shortly after, Jake was confronted by a policeman where someone had reported that Jake's party was causing a public disturbance, and was ordered to shut down the party or be arrested. Jake realized he couldn't escape, as he was not a kid anymore, and must convince the police to stop. Unable to stop the party, the policeman was about to bring Jake to the station when suddenly, Trixie broke her hips after trying to "dance her booty" one more time. Things got worse when the Gnome that was after Spud shot soda throughout the suite. Jake was overwhelmed of the responsibilities he had to face and stressfully fled the scene.
Outside, Jake was having a hard time realizing that the life an adult isn't what he thought it would be. He then received an urgent message from Fu Dog to come to the shop immediately. Jake discovered that Lao Shi's youth had been stolen and is growing very old. Fu dog then told Jake that if he doesn't vanquish the Avemetrus soon enough, Lao Shi would continue to age until he died.
At his home, Jake called for Haley's help to lure the Avemetrus for her youth. When the Avemetrus arrived, the two engaged the creature. Haley got grabbed by one of its tentacles, but before it could steal her youth, Haley bit its tentacle. They were both painfully thrown into a kid-stuff warehouse. Jake then devised a plan, thanks to Haley, of the Avemetrus stealing its own youth. They harassed the Avemetrus with toys and noises of arcades. Jake and Haley then landed on its back. The Avemetrus tried to fend them off with its tentacles, but this resulted in it sucking out its own youth. It then went out of control until the Avemetrus was finally defeated, and it exploded. Jake, Trixie, Spud, and Lao Shi's age are restored.
The next day, Jake caught up to Trixie and Spud, where he was in bad condition after being forced to do hard work just to pay off his credit card debt. Trixie and Spud were proud of Jake realizing that being a 14-year-old had its advantages. Brad was then seen driving past them, causing the water in the street to splash at them. While enjoying the life of being an adult now, Brad kicked the gas pedal into full speed with his new car and crashed.