"Lost Smurf" was an episode that appeared in Season 8 of the Smurfs cartoon show. It is considered that season's premiere episode.
As Nimble and Cadence, two traveling entertainers, are busy deciding which way they should go now that they have reached "the crossroads of the world", the blue sky suddenly starts shifting colors and a cloud appears before them. Then the clouds part, revealing a mysterious castle that appeared out of nowhere. Unsure of where it came from, but grateful to have a place to stop and have lunch at, the two entertainers enter the castle when two iron hands at the gate grab them and toss them inside. The castle suddenly comes to life, welcoming its new guests by referring to itself as Castle Captor, "where there's always room for more", before bellowing out in sinister laughter.
In the Smurf Village, as the Smurfs look in wonder at the shifting-colored sky, Greedy, Poet, and Painter are inspired by its appearance to work on their creations, while Brainy trips over a rock carrying a stack of his Quotations books to put in a safe place, believing that the sky's appearance is a bad omen. Elsewhere, Grandpa Smurf looks up at the sky along with Snappy and Sassette, the elderly Smurf being reminded of a terrible day that happened many years ago. The two young Smurflings eagerly want to hear about what happened that day, but Grandpa Smurf is rather reluctant to tell it, saying that it's a story that doesn't have a happy ending. Soon Papa Smurf comes along, wondering what all the commotion is about. After hearing from Snappy that Grandpa Smurf had a story that he didn't want to tell, Papa Smurf tells the Smurfling that he probably has a good enough reason not to tell it. But then the village leader sees that Grandpa Smurf has walked off into the forest, not wishing to tell anyone where he is going or even why. All Papa Smurf could surmise is that something very serious is troubling Grandpa Smurf.
Back at Castle Captor, the two entertainers are put to work mopping the floors of the castle along with a few other prisoners. Cadence is itching to get himself out of that place, but the other two human captives, a prince and his guardian, say that there's no escape from Castle Captor and that they have been inside it for 500 years, remaining un-aged during that time. The prince's guardian tells the entertainers that Castle Captor appears every 500 years to abduct innocent travelers to do his dirty work. The castle itself now speaks, saying that soon it will disappear for another 500 years with whatever and whoever is still inside it. Then a bell rings, and the castle notices that one of its prisoners is flying a kite, trying to escape again. The castle causes one of its gargoyle sculptures to snap the kite string, maliciously telling the prisoner to watch the hopes of escaping "fly away".
Out in the Smurf forest, Wild is at his tree house, also looking up at the unusual sky appearance when a kite lands on his doorstep -- the same kite that has flown away from Castle Captor. He then sees Grandpa Smurf passing by his way and drops down to show the elderly Smurf the kite he had found. Grandpa Smurf inspects the kite, noticing that it has knitting that he hasn't seen in over 500 years, plus some kind of message written in ancient script. Realizing who and where the kite and its message came from, but not willing to share this information even with Wild, Grandpa Smurf gets himself moving onward more quickly.
A short while later, Papa Smurf, Snappy, Sassette, and some of the adult Smurfs join Wild at his tree house, watching Grandpa Smurf take off in his magic balloon to wherever he is going. As the balloon sails against the rough wind, Grandpa Smurf prepares himself to say the magic words to get the balloon to travel where he wants it to go. However, he doesn't notice that an anchor on a rope has dropped out of the balloon and latched itself onto Wild's tree house, taking it and the Smurfs within along for the same ride. They notice that Grandpa's balloon is heading straight for the mysterious castle, but Hefty sees that the tree house is headed straight for a tree top, where the anchor detaches itself, leaving them behind outside the castle.
Grandpa Smurf wakes up to find himself at the intended destination, though he is rather surprised by the appearance of the castle. The balloon's anchor hooks itself around a pole, causing Grandpa to drop out of the balloon and into the paws of a living gargoyle statue, which then tosses the elderly Smurf into the castle through a window. Castle Captor greets its new captive as Grandpa Smurf flies through a series of closing doors before safely landing on a big chair. However, the castle's chair grabs him tightly and tries to put the squeeze on him. Grandpa Smurf escapes the chair, but finds himself outrunning a rolling carpet that captures him and dumps him down the stairs into a dungeon, where he meets up with "an old friend" of his -- an elderly female Smurf whom he calls Nanny.
Surprised to see Nanny again, Grandpa asks her where she disappeared to, and Nanny just answers that it wasn't her that disappeared, but rather the castle that did. She proceeds to scold Grandpa, saying that if he hadn't been so stubborn, she wouldn't have wound up in that place. This leads to the two elderly Smurfs briefly fighting with each other before they decide arguing over who's to blame for what happened is useless and that they better put their efforts into getting out of Castle Captor. However, Nanny isn't going to leave the castle without freeing her friends that are in it. After Grandpa Smurf uses one of Jokey's surprise boxes to get them out of the dungeon, he reluctantly follows after Nanny to rescue her friends.
Meanwhile, Papa Smurf and his group of brave Smurfs approach the castle's drawbridge in the hopes of finding Grandpa Smurf and where he disappeared to when the drawbridge suddenly closes up, causing them to slide inside. Again Castle Captor comes alive to malevolently greet its new guests. As Grandpa Smurf and Nanny escape being chased by a bearskin rug, an animated suit of armor trips over the same rug and causes Papa Smurf and his group of Smurfs to go flying onto a table, only to face an army of silverware that they flee from. They enter into a hallway with many doors and no idea of which way to go, which means that they must split up and search every room until they find Grandpa. As that group of Smurfs find themselves getting chased around by an ottoman, Nanny and Grandpa pass by them, though Grandpa is briefly distracted by what he gets a brief glimpse of. Soon a chandelier grabs those other Smurfs and dump them into a trap door that opens up.
Down in another dungeon, Nanny shows Grandpa her other friends -- the prince and his guardian, who are also joined by the two entertainers, who are now noticing that the walls of the castle are starting to vanish, meaning that the castle will soon vanish as well. As the humans flee from the dungeon, Nanny goes off to find "her special friend" while Grandpa bravely fends off a bunch of clawing chains ready to grab them.
In a dark place, Brainy wonders how "the six of them" got into this mess when Papa Smurf realizes that there's only five of them wherever they are in the castle. The sixth pair of eyes that are seen, as Hefty opens up a window, turns out to belong to a pink marsupial-type creature that identifies itself as a Smoogle. Papa Smurf calms down the scared Brainy by saying that this creature could help them escape, but that thought turns dark when a suit of armor enters the dungeon, the castle ready to put them to work as its slaves. Smoogle rolls himself up into a ball and attacks the suit of armor, toppling it to the ground. However, the castle shakes, and soon the Smurfs could see the walls starting to fade -- and them with the castle as well. Castle Captor tells them that soon they will join it for a "500-year vacation". Smoogle then points to a way out, and Papa Smurf tells his group of little Smurfs to follow the Smoogle.
Meanwhile, Grandpa and Nanny are hiding within a pot to avoid being captured by the patrolling suits of armor, with Grandpa ready to call it quits on finding her "special friend" and Nanny busy knitting up some decoys with which she uses Grandpa's roller skates to throw their pursuers off track. Soon a suit of armor follows the decoys as they lead it toward a staircase, where it falls down and doesn't get back up. Nanny notices a small high window they could use to escape the castle, if they could only reach it. Grandpa tells her to leave that part to him, and soon with her riding piggyback, Grandpa scales up the walls using his suction cups to reach where he has left his magic balloon. Just as they reach the balloon, one of the gargoyle statues try to attack them, though Nanny quickly ties up its jaws with her yarn, causing it to fall down.
Outside the castle, the four humans, whom Nanny and Granpda see as they prepare to take off, wave goodbye to the castle as it is slowly fading out of existence. But the two Smurfs don't get too far when they hear Sassette's voice and notice that Papa Smurf and his group of Smurfs are also at the castle, including Smoogle. They are trying to escape disappearing along with the castle as they climb to the topmost part of its roof. Nanny notices that her "special friend", which turns out to be Smoogle, is also down there with them. She suddenly takes control of Grandpa's balloon and comes to their rescue, but as close as they could get toward the pole that the other Smurfs are now on, Grandpa Smurf can't reach Papa Smurf...until Smoogle down below them extends his ears far enough to push the Smurfs upward.
Slowly the group of Smurfs climb safely onto Grandpa's magic balloon and grab Smoogle from off the pole to pull him in, but before they could fly off, a distraction causes them to loose Smoogle from their grip, only to fall into the wings of a bird statue. Castle Captor relishes in the thought that he at least has one of them. Nanny dives into the swirly-colored maelstrom to rescue Smoogle, while other Smurfs quickly form a chain to pull Nanny back into the balloon as he grabs her "special friend" from out of the soup. The pole's hand tries to grab the balloon to prevent it from taking off, but Grandpa hits the hand with his cane, releasing the balloon from its grip. Castle Captor swears that it will be back as it totally disappears into the mist that it came from. While the balloon sails away, Nanny is pleased to have Smoogle back with her and cuddles him close to her. Grandpa also introduces Nanny to the other Smurfs present, who then proceeds to tug at Papa Smurf's cheek just as she did Grandpa's earlier in the story.
Just as the balloon safely reaches the Smurf Village, they see flashes of light coming from where Castle Captor appeared, which Nanny says is just the castle disappearing for another 500 years, though Grandpa Smurf wishes it will disappear for 5 million years. With a final flash and a bang, the shifting-colored skies turn a calm normal blue again. Papa Smurf then proceeds to introduce Nanny to the rest of the Smurfs, who in turn introduces her friend Smoogle to them. They all gather around the elderly female Smurf to greet her, and some Smurfs like Painter and Poet present her with gifts of their own creation now dedicated to her. All Grandpa Smurf could say is that with Nanny in the village, things will never be the same.
- Unlike the previous two seasons, this episode, which was Season 8's premiere episode, was a single-part episode of standard running length. It would also become the last premiere episode that would introduce a new main character to the Smurfs cast, as Season 9 would shift the show's premise to time-traveling throughout the remainder of its run.
- Grandpa Smurf "breaks the fourth wall" in this episode when he whispers to the viewers about the surprise box he lays at the door of the dungeon in order to escape.
- The color-shifting skies are similar to the "red skies" effect that was used in superhero comic book stories such as 1985's Crisis On Infinite Earths, where such skies would portend a coming danger.
- Smurfette does not appear anywhere in this episode.
- Nanny's appearance brings up more questions than answers on whether female Smurfs can only be created or whether they can be naturally-born. Unfortunately, the cartoon series never addressed these questions and Nanny's origins (whether she was naturally-born like the male Smurfs or magically-created like Smurfette and Sassette) remain a mystery to this day.
- Continuity: as Grandpa's balloon carries Wild's tree house to Castle Captor, Snappy is seen among the Smurfs in the tree house, though he is no longer with them as they enter into the castle. However, near the end of the episode, he mysteriously re-appeared at the Smurf Village welcoming back the returning Smurfs.
- Character error: Nanny refers to Grandpa as "Grampy" even though when she last saw him 500 years ago, he was formerly known as Papa Smurf.
- Character error: Grandpa did not show any surprise that Nanny had not age at all in her 500-year captivity at Castle Capture.
- Character error: The Smurfs were surprised to find Nanny in Grandpa's magical hot air ballon after Grandpa called her name even though they would have immediately saw her when they went on board the gondola.
- Plot hole: Near the end of the episode, Papa Smurf was surprised of Nanny's existence even though she disappeared when he was a Smurfling of around 40-50 years old and would have at least remembered her.