The script can be read here but below is a synopsis.
The heroes are crouched in some lava flats watching something invisible approach across the lava, leaving only footprints. As it reaches the bank they charge forward only to discover it is the Dungeon Master who has come to tell them that they can return home if they find the River That Rains Upside Down. A mystical river that flows up into the sky once a year for a minute only and will take passengers wherever they want to go. He instructs them to follow a nearby path before warning them never to touch the "beauty that breathes the beast". He then vanishes.
They follow the path but it forks so the heroes decide to split up. Hank, Sheila, Bobby and Uni take the left path while Eric, Presto and Diana take the right path.
The group taking the left fork soon enter a jungle where they encounter a giant iron golem who grabs Bobby and Uni. The golem tries to crush them both in the palm of his hand but Hank fires an energy arrow that sweeps them out of the hand and ferries them through the air, as though riding a javelin. Hank then fires further arrows to weld the golem's ankles together. Sheila then goads him into approaching her whereupon he falls over and breaks into its component parts. Sheila wonder what it was guarding and they climb onto its head to get a better view and spy a desert.
Meanwhile the group taking the right fork come to a river where some flowers are growing on a vine around a tree. Diana picks one of the flowers and Eric takes it from her, boasting that it is nothing compared to the one's that grow in his mother's garden. Suddenly Eric is enveloped in a cloud of gas and when it clears he has been transformed into a bogbeast much to Presto and Diana's amusement. Initially he doesn't understand what they are laughing about and goes to drink from the river where he spots his reflection but thinks it is an underwater monster. Rushing to warn his friends he eventually realises that it was his own reflection and begs Presto to use his magic to turn him back.
Presto casts a spell that changes Eric's clothes into a frock and a wig and then trying again conjures a potion that changes Eric into a fire-sneezing dragonish creature when drunk. His third spell zaps Eric back into a bogbeast but leaves him scorched. Fearful of the next spell Eric tells Presto to stop. Dungeon Master then appears and scolds Eric for ignoring his warning (even though it was Diana who picked the flower) and adds that he cannot reverse the spell and that the spell must be reversed soon otherwise Eric will remain a bogbeast forever but that one solution lays in what Eric dislikes the most. Dungeon Master then disappears. Diana and Presto speculate that what he hates the most is being a bogbeast so the company of other bogbeasts may reverse the spell.
Meanwhile in the desert Hank, Sheila, Bobby and Uni are parched. Bobby spots a watery mirage and Uni dives in only to find it is sand. Hank's bow begins acting like a divining rod so he fires an energy arrow, thinking it will point out where there's water. The arrow goes up into the sky and then comes back down and blows up the sand, creating a sink hole. The heroes slip down the sink hole and into the river bed of a subterranean valley.
Bogbeasts surround them with fishing nets and spears and Bobby shouts which startles them and sends the creatures running to hide. The heroes and the bogbeasts talk and settle their misunderstanding. The bogbeasts were fishing in the river and thought that the heroes were sent by the ogre Kawamung to scare the fish away. Kawamung wants to dam the river so that the bogbeasts starve and look elsewhere for food. Despite their numbers the bogbeasts are cowardly which explains why they won't battle Kawamung.
Meanwhile Eric, Presto and Diana are trekking through a muddy bog and are attacked by a serpent. Fleeing it they begin sinking in quicksand and disappear under.
The bogbeasts explain that they won't fight Kawamung until their leader arrives. He is written about in their sacred scrolls, being like them but not like them and appearing from the sky. On cue Eric, Presto and Diana drop into the valley and the bogbeasts presume that Eric is their leader. They swap stories and Eric agrees to lead the bogbeasts into battle against Kawamung if they help change him back to normal. They also discover that the dried-up river the bogbeasts fish in is the River That Rains Upside Down.
The heroes reach the dam with the ramshackle bogbeast army and are told that they need to confiscate Kawamung's medallion to defeat him. At the dam there are dwarf slaves making weapons to drive the bogbeasts out of the valley.
Sheila distracts Kawamung while Hank fires an energy arrow that tears the medallion from around his neck and ricochets it back into Hank's hand. However, it is a fake medallion and Kawamung mocks them, revealing the real but previously invisible medallion still around his neck. Kawamung then fires an energy beam from the medallion at Hank, missing him but incinerating a nearby tree.
The bogbeast army attacks but Kawamung fires another beam from his medallion that encases some of them in crystal. An invisible Sheila tries to steal the medallion but is caught by Kawamung who removes her cloak of invisibility. Bobby knocks a tree down on Kawamung but it misses and Kawamung responds by firing a beam at him that Eric intercepts with his shield and reflects back at the ogre; hitting him squarely on the medallion and making him drop Sheila.
Eric urges the remaining bogbeast army to attack but they run off.
Presto conjures a Frisbee but thinking it useless throws it away. The Frisbee jams in the dam's mechanisms and sabotages it, causing the dam to burst and the river to be restored. The heroes see the river flowing upwards like an inverted waterfall and realise that this is the way home.
Kawamung has grabbed Sheila, Bobby and Uni so with Hank distracting him by firing arrows Diana pole-vaults onto the ogre's shoulder and removes his medallion. With it gone he transforms back into a bogbeast and runs away, chased by enraged dwarf slaves.
Diana speculates that if the medallion can change that bogbeast into Kawamung the ogre then the medallion can also change Eric from being a bogbeast.
The heroes plus Uni jump onto a log that's floating by and ride the river, ferried upwards and emerging in an ornamental pool in the fairground. Eric puts the medallion on but nothing happens so decides to go back into the Realm. His friends follow not wishing to leave him there alone. Reunited with them in the valley Eric changes back to his normal self and Dungeon Master explains that what Eric disliked the most was being in the Realm.
A bogbeast jokingly commiserates Eric for becoming so ugly.