Moxette and Clumsy are in the forest fulfilling a task for Papa Smurf when they stumble upon an old fountain. Clumsy recognizes it and tells her that a mean Goblin named Mystico is trapped inside it, so they shouldn't touch the golden goblet on the pedestal behind him. As if on cue, he turns around and knocks it into the fountain, thus freeing him once more. This time, Mystico is out for revenge on the Smurfs and wreaks havoc on the entire village. Will Moxette and Clumsy be able to save the Smurfs and imprison the dastardly creature or will they all be turned into Goblins?
On one fine day strolling through the forest, Moxette and Clumsy have been sent out to gather ferns for Papa Smurf. They find a nice patch and begin to pick the plants for their baskets, and then after pushing aside a small bush in her path, Moxette notices a strange and lovely fountain made of crystal in a clearing. Curious, she approaches it and calls her friend over to see it; he recognizes it from a previous expedition and explains that a "mean ol' Goblin named Mystico" is trapped inside.
She asks for him to elaborate, so he tells her that he accidentally knocked over the golden goblet on the pedestal behind him into the fountain, thus freeing the creature. Then he went on a rampage turning everyone into Goblins until Nobody Smurf put him back in and saved them all - "that's why Nobody is actually Somebody Smurf now," he finishes.
Moxette criticizes Somebody for his new name, considering that Papa said he had the ability to hide, and says, "Why didn't he become 'Hider' Smurf or 'Hidden' Smurf or something?"
Clumsy admits he doesn't know and turns around, adding that they should stay away from the fountain. Suddenly he trips over the pedestal and knocks the goblet towards the fountain, which his companion tries to stop with her foot, but it slips out from under her shoe and lands in the water. They witness the arrival of the fearsome Goblin as he rises out of the waves and stands on the tallest crystal with his wings extended. They exchange glances as he cackles and drops down next to them with a wicked grin.
"Free at last," Mystico shouts triumphantly, "I can finally exact my revenge on you horrible little blue creatures!" He tries to zap them, but Moxette grabs Clumsy's hand and dashes away, so he takes to the air and follows them. She tells Clumsy that they must warn the village, but then he trips on a rock and Mystico lands in front of him - he does not hesitate to turn him into the big-footed Goblin he once was before, and then snickers as he continues trailing the female. He is eager to find the village and create a mess of trouble.
Moxette makes it back and immediately seeks out Papa Smurf who is currently talking to Brainy and Hefty; she simply tells him that Mystico is back, knowing he will understand what it means. He becomes greatly alarmed, as do Brainy and Hefty, and instructs them to alert the other Smurfs. Before she leaves, he asks what happened to Clumsy, and she says he has been turned into a Goblin - with that, she departs to spread the troubling news before the villain arrives.
Mystico climbs on top of one of the houses and skims the villagers as they panic and run in all directions; he laughs and jumps down, cutting off several Smurfs and transforms them. Then he leaps away and pokes his head through a window to another house, scaring the inhabitants out into the open, which allows him to transform them, too. He manages to do this to a great deal of Smurfs until he has turned nearly everyone into Goblins, but he spots Moxette and Papa escaping the village limits and gives chase.
They split up, so he follows Papa Smurf, knowing that he is the leader, and intends to get the female later. Papa decides to pay a visit to Ripple with the hope that he will be able to help distract the Goblin while his other little Smurf retrieves the goblet. He soon finds him while he is out with Karma and flags him down, but Mystico turns him into a Goblin from behind. The Woodsprites witness this and rush to him, but Mystico departs before they come close enough.
Meanwhile, Moxette races back towards the fountain, remembering that Clumsy told her Somebody put the goblet on Mystico's head to defeat him. She bumps into Clumsy along the way and hugs him, saying she's happy to see him - even if he's a Goblin for the time being - and then runs up to the water's edge and fishes out the goblet.
They make their way back to the village and she searches for the unruly creature; eventually she spots him and waits for him to stop flying. When he does, she signals her fellow Smurf-Goblins to surround him while she mimics a pig's devilish squeal to startle him.
Mystico sees her and smiles, raising his arms, but suddenly realizes he is surrounded and that she has the golden goblet in her hands. Terrified of it, he steels away from them and heads back into the forest, but she's hot on his tail. She threatens to put it on his head unless he goes back to the fountain on his own, so he unhappily heads in that direction.
Once there, he thinks he can take to the trees and sneak up on her; unfortunately, the other Smurf-Goblins are waiting above and watching his every move. Then he sees Moxette and starts walking backwards as she steps forward.
He begs her not to subject him to his watery prison and suddenly knocks his foot against the rock lining, which causes him to fall back into the water. At that moment, Moxette charges and thrusts the goblet on his head, so the water rises up and claims him just like before. As he disappears, he lets out one final cry: "I'll be back, Smurfs! You haven't seen the last of me!"
The moment the waters calm and the cursed Smurfs are returned to normal, Ripple and Karma appear with Papa Smurf. Ripple asks what happened to Mystico, and Moxette explains that, with teamwork, she was able to defeat him. He looks into the water rather mournfully and says nothing. His girlfriend offers him comfort while the Smurfs cheer for Moxette and Clumsy since both contributed to the Goblin's downfall.
The episode concludes with her admitting, "I know Mystico's evil, but he's sort of cute - for a Goblin," and her friends laugh, taking it as a joke.
- This was originally episode ten in Season 10 of the Expanded Animated Universe. After the separation of the LD Stories, it was moved to its current position.
- Once again, Clumsy is the first Smurf turned into a Goblin by Mystico.