Based on the board game of same title.
Hank, Presto, Eric, Diana, Sheila, and her younger brother Bobby all go to an amusement park one night. While there, they go on a ride called 'Dungeons and Dragons.' But while on that ride, they get sucked into the world of Dungeons and Dragons. There they are attacked by various monsters and entities. When all seems lost, they're greeted by a short, mysterious man known as Dungeon Master. He gives each one of them a magical item that they can use to fight monsters and defend themselves.
Hank (the Ranger) is given a bow that creates arrows of energy. Presto (the Magician) is given a hat of magic spells. Eric (the Cavalier) is given a shield that can deflect any attack against it. Diana (the Acrobat) is given a extendable javelin that allows her to do feats of acrobatics. Sheila (the Thief) is given a cloak that when the hood is pulled up, makes her invisible. And Bobby (the Barbarian) is given a club that can crush anything it is struck against.
They're all being pursued by the evil Venger, a man who's history is somehow tied with the Dungeon Master. His ultimate goal is to acquire all six magical items the kids possess. He has a sub-servant known as the shadow demon who spies on the kids and gives reports on their goings-on. Venger has a mortal enemy, a five-headed dragon named Tiamat. The kids are constantly looking for a way back to their home, but also go where the Dungeon Master tells them their presence is required.