Under Duck is the tenth episode of the first season along with the episode All Fall Down. Under Duck first aired on April 23, 2004. The episode is written by Joe Fallon and storyboarded by Greg Hill. Fish, snails, and a salamander make their debut in this episode.
The episode starts out by showing a closeup of a grassy hill of which Quack walks onto. The narrator begins contrasting Quack with a orange fish in Quack's pond. Quack jumps into his pond and begins to laugh which draws the attention of the orange fish. Later in the day, when Quack is swimming and singing, the orange fish follows Quack and questions what Quack is and pokes him. Quack is startled by this and runs off into the distance and arrives at Peep's can where Peep and Chirp are sorting leaves. Quack begins screaming and running in circles which causes Peep to ask Quack what Quack is screaming about. Quack then tells Peep that something cold and wet and then heads back to his pond. Peep and Chirp follow Quack and find him standing at the edge of his pond.
Quack looks under the water and notices the cold and wet creature has disappeared. Peep asks Quack where he saw the cold and wet creature and Quack reenacts what happened. While Quack is talking to the orange fish notices him and goes up to Quack and pokes him again. Quack screams and jumps out of his pond into Chirp's arms. Chirp collapses and suggest to Quack that he looks under the water and Quack dose so. Quack notices the orange fish and questions it. The fish explains that it is Quack's neighbor which Quack quickly denies and orders the fish to leave his pond. The fish laughs at this and swims away leaving Quack confused.
Quack jumps out of his pond and tells Peep the creature that was cold and wet was a fish. Quack walks away and the camera cuts to Quack looking at a tree. Quack returns to his pond and calls for the orange fish. When the fish appears Quack tells how he found a tree for the fish to live in. The fish explains that it needs water to live in. Quack leaves his pond and returns with a cup which he gives to he fish for its "new home". The fish then explains that it needs food and room to swim. Quack swims after the fish and discovers more creatures lurking on the floor of his pond. Quack asks the creatures what they are doing in his pond and the orange fish explains that they live in Quack's pond.
A small red fish tells how he recognizes Quack causing Quack to turn around. The orange fish then introduces Quack to the rest of the fish who crowd around Quack in excitement. Quack then tells the fish that they have to leave his pond and the orange fish responds. While Quack and the orange fish are talking the small red fish interrupts and tells Quack that they want to learn about ducks. Quack happily agrees to tell them about ducks and the pond's inhabitants gather together to listen. After a short while about hearing about ducks, the orange fish interrupts and tells the rest of the fish that they are bothering Quack and should leave. The fish feel upset about this and beg Quack to let them stay. The fish begin to leave but Quack then decides to let them stay. Quack resumes telling the fish about ducks while, above the surface, Peep and Chirp take note of how Quack has been gone for a long time. Quack suddenly pops above the surface and begins to sing. The fish sing along with Quack and the episode ends with Quack complimenting the fish on their singing.