The script can be read here but below is a synopsis.
Eric is in his underpants with his clothes strung up on a washing line and his armour discarded. He has been sprayed by a feathered skunk thinking it was a striped chicken. Presto conjures a gas mask as a solution.
Suddenly there is an earthquake and the ground splits open with Hank tumbling inside the fissure. The heroes enter the fissure and discover a padlocked chest. Eric wants to open it commenting that it probably contains "untold riches" but Dungeon Master appears and counters that is more likely contains "undead witches". (In the original script the dialogue was "gold and jewels" countered by "mould and ghouls".) He explains that it is Zandora's box and that Zandora was a good sorceress who was banished to another world many years ago by Venger. Empathising with Zandora being lost in another world the heroes ask how to free her and Dungeon Master gives them a map, telling them to open it at high noon under the shadow of Skull Mountain, but warns that they must never open it anywhere else. He then disappears behind a rock.
Using ropes (presumably conjured from Presto's hat) the heroes haul the chest out of the fissure but are observed by shadow demon.
In a jungle the heroes are ambushed by bullywugs who open the chest, thinking it contains treasure, and enter it. Eric slams the lid shut thinking they are trapped inside but when they open it later the chest is empty with the bullywugs transported elsewhere. Diana deduces that the map is a passageway to other dimensions and that whenever the box is placed on any of the map's spots it opens a stairway into another world.
Meanwhile shadow demon enters Venger's castle to find him chastising ice-encased monsters who he accuses of deceiving him. He tells Venger that the heroes have found Zandora's box.
In the jungle Uni spots what she thinks is Skull Mountain. The heroes query why it is closer than indicated on the map but are prompted to act when the noon shadow of the mountain's skull appears. Placing the chest under the shadow they open it and a staircase appears. The heroes descend with Eric and Presto remaining above. Watching from one of the mountain's skulls, shadow demon chuckles.
Diana likens the new dimension they have entered to the "twilight zone" and they hear a female voice beckoning them down. They continue and find themselves on a giant chequered board with a backdrop of stars and planets. In the middle is a grandfather clock that chimes, opening up the black squares and sending the heroes plummeting down into a caged pit.
Eric and Presto hear their cries but Eric is reluctant to help, suggesting that his friends are just checking out the echo.
Hank fires an energy arrow at the cage that acts as a rope and grapple and swings to safety, catching Sheila. Diana uses her acrobatics to grab the cage, as does Bobby who catches Uni by sticking his club out so that she lands on it. A giant wasp emerges from below.
Hearing further distressed cries Presto and Eric investigate. Shadow demon flies down and slams the chest shut, which causes the staircase to collapse. They both fall into the caged pit, and landing on the giant wasp jump off onto the bars in fright. Presto tries to conjure some bug spray but instead conjures another giant wasp. Hank binds the giant wasps' wings with energy arrows and then fires another arrow at the staircase remnants as a rope and grapple. They climb back up and Bobby uses his club to knock the chest lid open, causing shadow demon to flee.
The heroes wonder why the map was wrong and Dungeon Master appears to tell them it was an illusion just as the illusory Skull Mountain disappears.
Shadow demon reports the failure back to a displeased Venger, who orchestrated the illusion.
The heroes continue through a desert and find the real Skull Mountain. They place the chest under the high noon shadow and open it, releasing Zandora in a puff of blue smoke. She thanks them and wishing to return the favour teleports them to a natural bridge which is the spot the chest needs to be to return the heroes home.
Leaving Uni with Zandora they open the chest and enter. Then staircase disintegrates and the tumble through a void only to land in a Dungeons & Dragons ride car which shoots off through a tunnel of light.
Zandora bids them farewell just as Venger arrives on nightmare. They battle each other briefly but Venger manages to jump into the chest.
The heroes are elated as they arrive at night in the theme park but their celebration is interrupted as Venger attacks. Eric runs to a phone box to call the police but remembers he has no dime. The phone box is then destroyed by one of Venger's mystic bolts. Venger demands the totems but the heroes refuse and attack, only to find that their totems are powerless on Earth. Venger destroys a Ferris wheel and hot dog stand.
The heroes face a dilemma. If they refuse him the totems Venger will destroy their world but if they surrender the totems Venger will return to the Realm and conquer it. Hank comes up with an alternative, as they led Venger here they must lead him back. Jumping into the Dungeons & Dragons car, with a reluctant Eric finally joining them, they return back to the Realm with Venger in pursuit.
With everybody back on the natural bridge Zandora crosses to the other side and levitates the totems and the chest to her, before placing the totems in the chest and closing it. Venger fires a mystic bolt at Zandora but hits the chest instead, sending it flying.
The natural bridge collapses from a quake and the heroes flee to the other side to join Zandora. Venger opens the chest and enters, thinking he can claim the totems. Closing the lid Zandora moves the chest to where it was before the bolt struck and opens it to reveal the totems, explaining that as the chest had moved Venger had entered somewhere else instead.
Venger is in a cavern with a giant door. He opens it and finds to his horror Tiamat the dragon. Presumably this is Tiamat's lair.
Zandora thanks the heroes for freeing them and commiserates them over the natural bridge's collapse, saying that it was the only spot that the chest could have transported them home from. Eric refuses to acknowledge this and pushing the chest to another spot opens it only to release the bullywugs from earlier on. He slams the lid shut on them as Dungeon Master arrives to share the mirth with the others.