When the scientists accidentally drop a stupidity potion into the lake, every creature in the forest becomes infected, except for Daggett. Feeling left out, Daggett attempts to drink some of the lake's water as well, though not only he discovers that he's immune to it, he discovers that the gas he produces after consuming the water is the antidote which turns those infected back to normal.
Scientist #1 and Pete are in the forest trying to find bug spray to rid mosquitoes. Scientist #1 finds a bottle of Stupidity Potion #237 which causes overwhelming stupidity in 9 out of 10 creatures who drink it. Scientist #1 disposes of it by tossing it into the lake making the cork open and turn the entire lake into a rusty orange color. A fish swimming in it is the first one to catch it and it also makes Barry stupid as well as the fish walks inside him. A bored frog also drinks it as the potion in the water spreads like a disease.
At the dam, Norb wakes up at the sound of loud noises and sees Dag spinning the entire living room on a record player (he calls it the Lazy Suzanne) and acting like he is on a game show. Dag asks Norb how it is and Norb answers that question by mocking him for his stupidity. The record player, from Dag stopping the living room to a halt, begins to go really fast as Dag tries to stop it. The spinning room knocks off the furniture enough for Norb to also fly into the kitchen sink with orange water pouring onto his head making him stupid as well. Dag believes he is making fun of him again.
Scientist #1 and Pete come in and shoot vaccine balls at Dag thinking he drank the potion. Scientist #1 and Pete leave and put signs all over the forest. But it is too late now that every single animal in the forest have already caught it. The animals do strange activities like hitting each other with rocks, trying to milk bulls as cows, and walking through signs. Dag sees that everyone is acting quite stupid. Seeing the sign Scientist #1 put in his way, he goes a little brain-dead and says "That sign is making everyone stupid!" Scientist #1 then reminds him "It's not the sign; it's the water". Dag then says "Like I said, the water is making everyone stupid!"
Norb then runs outside with the other animals. Dag believes that he himself is the smartest guy around: "The supreme smarty-pants smartest guy!" Norb twangs Dag's nose and laughs in a goofy way. Dag says "Laugh all you want, no-brain, beaver. You'll all do my bidding soon enough, you mindless poopyheads!" Norb repeats the word "poopy" finding it funnier for him. Dag then orders all the animals to do as he says until Barry knocks Dag on a log into the mud where the other animals mess around with him. Tired of being smart, Dag decides to be like the other animals by drinking the water. Dag still feels the same since he can't get any dumber.
Suddenly, Dag belches right into Barry's face. Barry turns back to normal and then tells Dag "Whoa, baby, you got the breath that'll make a dumb bear smart". Scientist #1 and Pete see that Dag is not only the only one out of ten creatures of the potion who is immune but also an antidote. The two scientists put Dag into a spray gun making him belch on all the creatures back to normal and almost destroy the pine trees, except for Norb, who is seemingly avoiding them on purpose. The scientists load up Dag in order to revive Norb but they are stuck on the Lazy Suzanne as Norb tosses a barrel at them.
They load up Dag again but the force sends Dag in the lake. Scientist #1 claims that swimming and drinking in the polluted water would either make Dag a hundred time smarter or dumber or to Scientist #1's enjoyment, he would drown. A huge wave rises from the lake and something big rises out. Scientist #1 says "We should've known that weasel can't get any dum-ber!"
Back at the dam, Dag, now a hundred times smarter and his brain about the size of the moon, is using the scientists for stimulus response. In a British accent, Dag says "With my super brilliance, it appears I have now solved every problem in the known universe! Except for one: whatever happened to Norb?" In a Mesopotamian-themed country, Norb being praised as a god. Norb, still affected from the potion, is served fruit and still saying the word "poopy". The servants copy the word and praise him as Norb tosses the fruit to his servants.
- Barry rolls a log Daggett is on backwards. The log rolls down, a splash is heard, and orange drops fly into the air. But when the camera pans to show Daggett, he's sitting in a mud puddle.
- Marathon Man - The episode title is play on the title of the 1974 novel and 1976 film.
- Lazy Susan - The "Lazy Suzanne" Dag made in the living room is a parody of the rotating tray/platform.
- Nick Bakay as Norbert, Pete
- Richard Horvitz as Daggett
- John Garry as Barry
- Ed Winter as Scientist #1
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