The script can be read here but below is a synopsis.
The heroes are trekking through some misty mountains when they hear baleful moans. Eric is worried that there's something bigger and meaner making it moan and doubting that they'll find a way home turns back but bumps into Dungeon Master who tells them that a heartless dragon dwells in the Vale of Mists and when its heart is in the right place it may show them the way home. He then disappears.
Eric changes his mind and continues with the rest of the heroes who stumble through the fog before tumbling down into the Vale of Mists. From behind some boulders they observe gnome slaves mining gems from the vale's bejewelled walls. Two stone colossi stand over the village and orcs are on guard, running the mining operation. In the middle is a dragon statue on a plinth with a socket where its heart should be.
A gnome collapses from some scaffolding and protests that he is too weak to continue the mining. An orc chieftain cracks his whip and tells him there will be no rest until all the mystic gems have been mined for Venger.
Bobby rushes out and attacks two orcs, shattering their swords with his club and sending them fleeing in terror. He goes to the aid of the fallen gnome but more orcs converge and the remaining heroes intervene. Notably Presto changes an orc's armour to Hawaiian shirt and Bermuda shorts and his sword into a ukulele while an invisible Sheila ties the orc chieftain up with his own whip.
They urge the gnome to escape but he refuses, explaining that the vale is under Venger's spell and if any gnome leaves then they will die. He adds that only Lukyan the spellbinder can negate the spell and that he is imprisoned in the Swamp of Sorrow after refusing to tell Venger the secret of the dragon's heart.
The gnome slave is brought before Venger who questions him about what happened and then instructs shadow demon to go to the swamp and spy on the heroes, on the chance that Lukyan will tell them the secret of the dragon's heart.
Meanwhile the heroes are trekking through the swamp and they meet Dungeon Master who tells them that Lukyan is in the prison without walls, before vanishing. The heroes are then attacked by violet fungi but then a shambling mound rescues them. The heroes attack the shambling mound thinking that it is hostile and Hank fires an energy arrow at it but this just causes the creature to grow in size. Presto then conjures some weed-killer spray. Hank sprays the shambling mound with it and the creature retreats.
The heroes trek further through the swamp and meet Dungeon Master who says that they will recognise Lukyan by what he spoke without words and that they must do so before the crossing of the four suns, before disappearing in some reeds. Shadow demon overhears all this.
The heroes come across a ramshackle hut and wonder if it is the prison. Inside they are attacked by zombies but rescued by the shambling mound who demolishes the hut. Scared by its approach the heroes are about to attack the shambling mound again when Hank realises that it is Lukyan, as his rescuing actions speak without words. The shambling mound nods emphatically and beckons them to follow it.
The shambling mound shows them magical artefacts stashed inside a tree's hollow, including a glowing purple gem, the heart of the dragon, and a wand. Hank deduces that Venger transformed Lukyan into a shambling mound to prevent him from performing the gestures and incantations needed for magic. Diana adds that the shambling mound form is a prison without walls. Presto helps cast a spell and transforms the shambling mound back into Lukyan. Lukyan teleports them back to the Vale of Mists. Shadow demon has observed everything.
Lukyan and the heroes appear in the vale on the dragon statue's plinth. Lukyan negates Venger's spell and traps the orcs. Eric asks how to get home and Lukyan places the gem in the dragon statue's empty socket just as the four suns eclipse, sending a ray of light onto the dragon heart that refracts onto the bejewelled walls and creates a holographic display of the cosmos. Lukyan adds that each point of light is a gateway to another world.
Venger arrives on his nightmare mount and having learnt the dragon heart's secret announces that he has no further use for Lukyan or the gnomes. He animates the two collosi into stone golems and they attack, stomping all over the gnomish huts.
One of the golems grabs Uni but, encouraged by Bobby, she teleports to safety.
Lukyan and Venger battle each other with mystic bolts with Venger freeing the orcs from their prison.
Presto conjures a cannon but without any cannonball so attempts to conjure one but produces ball-bearings instead. One of the golems slips on them and shatters.
Bobby bats the cannon at the remaining golem and shatters it.
Lukyan manages to banish Venger temporarily and when the heroes ask him which jewel shows them the portal to their home he apologises and explains that the portion of valley wall where the gem was was destroyed by a stray mystic bolt.
Dejected the heroes leave the vale and Eric wonder whether they will ever find the way home. Dungeon Master appears and says they will in time. Eric counters with "In time for what? Our retirement?"