The episode starts at the Smurf village where Papa Smurf tells his little Smurfs that they have to rebuild the bridge to cross the River Smurf. They begin to work, of course, but they have difficulty due to Brainy's constant lecturing and Jokey's pranks. Joey, Moxette, and the Pixie trio are helping as well when they realize they are out of wood. Papa Smurf tells Pansy and an unnamed Smurf to go into the forest for some more, so they lead away.
They are cutting down some trees when a Purple Fly swoops down and bites the Smurf's tail, thus turning him into a "Purple Smurf." Pansy flies away from the Purple Smurf and tells Papa back in the village that, "The Smurf was bitten by a fly and turned purple with a mean-looking face, and now he only says "'Gnap'!"
Papa is very concerned and says, "Oh no! I can't believe it's happening again!" He alerts the rest of the village about the Smurf who was bitten by a Purple Fly and they all gasp.
Not knowing about this threat, Joey asks, "What do you mean 'bitten by a purple fly again'?" Papa explains to Joey, Moxette, and the Pixie trio that long ago, before he met them, Lazy was bitten by a Purple Fly and turned into a Purple Smurf. While he was trying to find the cure, many other Smurfs were bitten by the Purple Smurfs and turned into one themselves.
Until he found out the cure was a "Tuberose," it was a hard battle to fight, and eventually he became the last Smurf. He was then bitten and turned into one too, but luckily an explosion of Tuberose's pollen turned all the Smurfs back to normal.
Papa Smurf turns to the others and encourages them to capture the Purple Smurf - as well to find the Tuberose. Two Smurfs, Clumsy and Forgetful, were searching for the Purple Smurf in the forest.
Forgetful notices someone behind him, but forgets what he was supposed to be doing. Then he is bitten by him and turns into one. Clumsy tries to catch them, but accidently trips on a tree root and recieves a bite, too.
Meanwhile, Joey, Smurfette, Moxette, Hefty, and the Pixie trio are searching for the Tuberose when they notice four Purple Smurfs moving towards them. Joey manages to knock them out by using his suplex and ties them up. Hefty decides to take them back to the village; once they arrive, however, the Purple Smurfs break free from the ropes and bite Hefty, much to Papa's shock.
One by one the Smurfs are being bitten and transformed, so he hides Baby Smurf inside a basket, and tries to warn Grandpa and the Smurflings, but he comes too late - they were already bitten earlier. Grouchy tries to run away and hide behind a bush, but the leader sneaks up and bites him. Before transforming he says, "I hate Purple Smurfs!"
Meanwhile, Joey and company continue looking for tuberoses; Moxette notices three Tuberoses and says, "There they are!" They go back to the village, and as they arrive, they are shocked to see the entire village completely Purple Smurfested. Before any harm comes to them, Papa Smurf - the last normal Smurf beside them - saves them from an ambush.
The Pixie trio decides to stay at the Great Oak until everything is safe when suddenly Purple Handy and Painter appear from behind and bite Joey and Smurfette's tails. Joey cries, "Oh no! I'm going to turn into a Purple Smurf," but nothing happens to him. Unfortunately, Smurfette turns into one and bites his tail - still nothing.
Papa Smurf exclaims, "Amazing! It seems you're immune from the effects because of your humanity!" Moxette wants to know about what he meant by that, but he says he'll explain later since they have to finish this crisis. They race to his laboratory and make some Tuberose's pollen.
Once inside, the army of Purple Smurfs approach the lab and the fight begins: Moxette, Joey, and Papa spray pollen at them and are relieved to see them turning back to normal. The cured Smurfs join in the fight until they run out of pollen. Papa Smurf tells them to retreat to his lab, but as they enter, the leader is already inside and bites Papa.
Moxette tells Joey to go ahead while she holds him off, so Joey asks, "But what about you?" She tells him not to worry and that she'll be fine, so he leaves to find the antidote. While he does, Moxette distracts the Purple Smurfs by mimicking their "Gnap" noises.
Joey knows that she will not hold them for much longer, and then he notices a torch hanging against a wall. Remembering Papa's story, he throws the pollen at the torch just when Moxette gets bitten by the leader and turns into a Purple Smurfette.
Suddenly an explosion erupts and the pollen covers the entire village: all the Smurfs are cured, Baby sneaks out of the basket, and the Pixie trio come out of the Great Oak. Papa Smurf is happy that peace has returned and everything is back to normal when Moxette says, "Almost..." They notice the Smurf with a mean-looking expression and he asks if is Grouchy.
The Smurf starts to complain: "Grouchy?! How can you mistake me for Grouchy?! And why does every Smurf wear white clothes?" Papa Smurf says, "This is the result of being a Purple Smurf for too long," and the other Smurf complains about Purple Smurfs.
Moxette asks, "What did you mean about Joey being human on the inside?" Having no other choice, Joey reveals that he is actually a human who was turned into a Smurf and lost his memory, and tells her that she has to keep this a secret.
The next day is peaceful in the village, but it doesn't last long because Jokey plays a prank on everyone by painting himself Purple. It causing panic to restore about the Purple Smurfs; however, the prank backfires when Handy starts asking around about someone stealing his purple paint - a paint that never comes off.
Meanwhile, the formerly unnamed Smurf (now called "Complainer") starts to complain about Jokey always pranking everybody, thus ending the episode.
- The scene where Jokey paints himself Purple is based on the one-page gag of the comic version with only two differences: in the comics, the black paint was changed to green and the word "Gnap" was changed to "Gnash." In the cartoon, he painted himself purple and still says, "Gnap."
- This is the first appearance of Forgetful Smurf.