The episode starts at the Smurf village where all the Smurfs are doing their daily chores: Handy is fixing things, Hefty's running around the village, Brainy's lecturing gets him booted out of the village, Fooly is fooling around, etc. All are having a good time while singing the Smurf Song until the Piximp appears and complains, "Why do we always sing this song? It's always the same thing! Doesn't any Smurf know a different one?"
Joey, Smurfette, Clumsy, and Moxette are shocked to hear what he said; they hear someone and look below. It's Stingle whistling and singing "Listen to the Mocking Bird," which makes Moxette say, "Isn't he that talkative Scorpomouse who helped us save Christmas?"
Meanwhile on the other side of the Smurf Forest, a dozen Dryad Nymphs are flying around and singing, but their beautiflul song is interrupted by an off-key voice. They stop and glare at a certain Nymph (revealed to be Siren) and tell her, "Siren, stop that!"
She asks why, so one answers, "You sing very badly," and another agrees with, "You cause pain on our ears!" Siren becomes offended by this and tells them that she is training her voice which makes them shout, "If you want to train your voice go somewhere else," so she sighs and leaves.
Later at the village, Papa Smurf tells his little Smurfs to gather some "Catnips" to create a cat repellent to prevent Azrael from approaching them. He chooses Joey, Smurfette, Moxette, and Harmony to go; as they leave they hear a voice yell, "Hey, wait for Fooly!" Fooly decides to join them, and while walking Harmony starts to play his off-key trumpet much to their annoyance (especially Moxette's), although Fooly seems to enjoy it.
Moxette tells him to stop playing and he asks her why, so she reminds him that he plays his trumpet very badly. As usual, he is offended and claims his song is a masterpiece - Moxette asks Joey and Smurfette if they feel the same way, but Joey and Smurfette (not wanting to upset him) start whistling until they hear someone singing a bad tune that even Harmony doesn't like.
They follow the horrible song and find out it is Siren, still training her voice, so they ask what she is doing. Upon seeing them, she becomes frightened and flies away; they search for her, telling her that they won't hurt her. Siren, who has turned herself into a small tree, hears this and turns back to normal to have a closer look. When she deems the Smurfs friendly, they introduce each other.
Harmony and Moxette ask why she was singing so poorly - considering she's a Dryad Nymph - and she tells them that since she was born, she has maintained a horrible singing voice. She's able to make glass shatter (in the negative sense), so she has been training until she becomes a decent singer. Joey and Smurfette decide to help her out.
Meanwhile, Gargamel and Azrael are searching for the Smurfs for dinner: they give up after a while, unsuccessfull, until they hear a horrible tune. They go to investigate what it is and find Siren still singing off-key. Harmony says, "Ooooow...my head is spinning..." and Moxette agrees with, "I think this song is going to drive me Smurfy!"
Suddenly they hear, "There's only one person who can drive you Smurfy - me!" The Smurfs cry, "Gargamel," to which Moxette comments, "So that's Gargamel." They run in all directions except for Fooly who stands staring stupidly at Gargamel; Moxette returns and pulls him away.
Joey attempts to drive Azrael away, but the feline manages to whack his sword away. Luckily, Moxette saves his life by mimicking a dog's bark which scares Azrael. Gargamel tries to catch Harmony and Smurfette with a bug catcher, so Joey thinks of an idea: he tells Siren to sing to which Moxette says, "Is this a good time for that?"
Joey tells her to trust him, and then Siren starts to sing - Gargamel covers his ears and drops the net. After a minute, he can't take it anymore and runs away while blaming the Smurfs for another failure.
Later, the Smurfs thank Siren for saving them. She tells them she is going to train until she gets better as she flies away. They bid her farewell while waving and Harmony apologizes to Moxette and confesses to her that he now knows how she feels when he plays his trumpet. She forgives him, and so Smurfette says, "Well, at least everything ended Smurfy." Joey agrees with a simple statement: "They finally learned to settle their differences..." thus ending the episode.
- Stingle makes a cameo appearance when passing by Clumsy, Joey, Smurfette, Moxette, and Complainer; he whistles the tune of "Listen to the Mocking Bird."