"The Secret Of Shadow Swamp" is a Season 4 episode from the Smurfs cartoon show.
The Smurfs in the Smurf Village are preparing for the celebration of Grouchy's birthday as they are setting up tables, banners, and decorations for his party. Meanwhile, Grouchy is just standing there with his arms folded and his face scowling, hating the idea of celebrating his birthday. Papa Smurf tells Grouchy that he's going to have a birthday party whether he likes it or not and hopes that he wouldn't run away from it like he did last year. Grouchy just walks away to be alone with himself and his thoughts, though he isn't alone for long as Angel and Devil Smurf show up to argue about Grouchy's right to not have to celebrate a birthday. Then Jokey shows up to tell Grouchy that he has a surprise for him, but he will only give it to him at his birthday party. Grouchy shouts that he hates parties and storms off into the swamp, causing Jokey to drop his "surprise" and make it explode on the ground near himself. Jokey calls out to Grouchy, telling him that Papa Smurf warned them not to go off into the swamp and that the Smurfs are going to have a party for Grouchy with or without him.
Sometime later, Tailor asks Papa Smurf if he has seen Grouchy because he needed him to finish work on his new birthday suit. Jokey also comes along to ask about Grouchy, and Papa Smurf has a feeling that Grouchy has run off again, though he doesn't like hearing that Grouchy has run off into Shadow Swamp, which he says is the most dangerous place. He says that although Grouchy doesn't like parties, he's going to get a search party, and so Smurfette, Jokey, Tailor, and Nat volunteer to join Papa Smurf in their search for Grouchy.
Meanwhile in Shadow Swamp, Grouchy is just standing there looking at the dismal scenery when an imp-like creature comes by looking at a map and singing a song when he runs into Grouchy. The two of them stand there looking at each and grunting until the imp introduces himself as Dismal Boggs, showing with his map that he is looking for the Enchanted Falls, where he says he will be set for life. He describes the Enchanted Falls as a place with crystal-clear waters and delicious nuts and fruits, and even better, it is also a place where Dismal Boggs won't be bothered by anyone. He invites Grouchy to tag along with him, which Grouchy says he hates doing until he gets scared off by a spider and decides that he hates going alone.
As Papa Smurf and his team of four Smurfs enter the swamp to look for Grouchy, they soon spot other tracks which Papa Smurf recognizes as being made by an imp. He fears that Grouchy may have been led into trouble by an imp and so they rush after him, following the tracks he and the imp have made. Meanwhile, Dismal Boggs uses an extendable arm Dismal to pull a cluster of swampberries off a tree, which Grouchy tastes and spits out, saying that he hates swampberries. Dismal says that the swampberries aren't for them, but for the creature that lies ahead. Soon Grouchy gets stuck sinking in a bog until Dismal Boggs pulls him out of the bog and onto the rocks that he tells Grouchy to stick to. The two of them jump across the bog from one rock to another until Grouchy jumps onto a "rock" that turns out to be a hideous Swamp Creature. He is ready to have Grouchy for a snack when Dismal Boggs gives the swamp creature the swampberries to eat instead of Grouchy, distracting him long enough for them to make their escape. Papa Smurf and the four Smurfs with him also find out that the swamp creature is more interested in eating berries when they make their attempt to cross the bog and he gets his hands on one of their smurfberry pies.
Grouchy and Dismal Boggs soon come across a part of the swamp where they will have to get across by using a boat. Dismal sees a mushroom cap big enough to use as a boat and so cuts it down while Grouchy is again being bothered by Angel and Devil Smurf debating whether he should go with Dismal across the swamp and miss out on his birthday party. After they launch off into the murky waters in search of the Enchanted Falls, Papa Smurf and the four Smurfs arrive to see what Grouchy and Dismal have done and decide to do the same when the swamp creature following them shows up, demanding for more pies. Papa Smurf understands what the creature is saying and tells Nat to tell the creature he can have more pies if he will help them get across the swamp.
Soon Grouchy and Dismal Boggs reach the Enchanted Falls, with Dismal glad that they have made it and they are home free when their mushroom boat suddenly gets caught and pulled into a whirlpool. As Grouchy tries in vain to row their way out of the whirlpool, Dismal pulls out a fishing rod and casts it around a tree so that they can reel themselves out. Safely away from the whirlpool, they appear before the falls when a female figure that Dismal calls a Mist Maiden appears, asking them what it is that they seek. Dismal asks about the secret of Shadow Swamp, how they can get to paradise, and the Mist Maiden says that it's only through her that they can enter, but she warns them as they enter that there's no return once they enter that domain.
Past the waterfall, Grouchy and Dismal Boggs see a beautiful place with a crystal fountain that is the secret paradise that Shadow Swamp hides. Dismal decides that it's the perfect place for him to rest, but before Grouchy can do the same, Angel and Devil Smurf appear again with Angel warning Grouchy that his fellow Smurfs will come looking for him and that he would never forgive himself if anything happens to them. Grouchy decides that paradise isn't for him and heads for the boat to leave.
Dismal hears Grouchy voicing his thoughts on leaving and decides to let his traveling companion do what he wants while he decides to lie back and rest. But just as he lies back to rest, he hits his head on a stone which turns out to have a secret map underneath when he lifts it up. Dismal realizes that it's a map the Lost Valley Of The Vultures and decides that Shadow Swamp's paradise isn't for him either as he goes to join up with Grouchy on escaping the place. However, the Mist Maiden who guards the paradise sees the two adventurers escaping and tells them that there's no escape. Grouchy and Dismal find themselves struggling as she causes the waters to get rough and they end up caught in the whirlpool until a hand reaches out and grabs them. It turns out to be the swamp creature that is now helping Papa Smurf and his team of Smurfs to rescue Grouchy. As the swamp creature brings his passengers across the swamp, Papa Smurf tells Grouchy that he hopes his little Smurf will not cause his friends to worry about him, and Grouchy says that he hates causing his friends to worry.
After the Smurfs make their goodbyes to Dismal Boggs and the swamp creature, they return to the village in time to celebrate Grouchy's birthday party, with Papa Smurf getting his little Smurfs to cheer for Grouchy, saying he gives new meaning to the term "many happy returns". As the Smurfs cheer for Grouchy's safe return, Papa Smurf tells Grouchy that since his birthday day got such a late start, he doesn't see any problem with letting his little Smurfs stay up past bedtime. The Smurfs cheer again at this news, as Lazy is awakened to find this out and says that he hates celebrating Grouchy's birthday.
- The syndicated version cuts out a scene where Greedy, Handy, Lazy, and Harmony are outside at the table waiting for Grouchy to return. This included Harmony blowing on a party horn, which wakes up Lazy and says "Happy birthday, Grouchy!" before going back to sleep. As a result, the other three say the same, followed by Handy saying "Wherever you are".
- A scene after the swamp monster saves Grouchy and Dismal Boggs is shortened in the syndicated version, removing Smurfette's line ("Oh, Grouchy, we were so worried about you!") before Papa Smurf scolds Grouchy.
- A children's book adaption of this episode was released internationally, titled "The Smurfs and the Swamp Monster". It is one of few Smurfs Classic books that have not yet been released in the US.