The Trap Door

Berk (voiced by Willie Rushton) is an oviform blue creature who speaks with a West Country accent. He is the protagonist of the show, and steward or caretaker of the monster's castle. As such, Berk often goes about his duties with simple-minded glee, and enjoys cooking with ingredients such as mud, eyeballs, snakes, and worms. Berk is often warned not to open the trap door by his friends, or forbidden by his master, but often does so nonetheless. His usual exclamations include "Oh, Globbits!" and "Sniff that!"

Boni (voiced by Willie Rushton) is a disembodied human skull, and Berk's closest friend. Speaking with an upper-class accent, he is something of an intellectual, but has a tendency to complain or bore others. Boni dislikes to be moved from his favourite spot—an alcove in the wall near the trap door—and is often shown failing to warn Berk about the various monsters that come from it. Although serious most of the time, he is given to childish excitements on par with those of Berk.

Drutt (voiced by Nick Shipley) is Berk's pet, resembling an oversized spider, who often causes trouble when chasing after worms and other invaders, as by passing the trap door in search thereof. Although often characterised as male, Drutt produces a litter of baby spiders in the show's second season. The voice of Drutt is that of Nick Shipley, then proprietor of KPTV, who provided the editing services for the early series of Trap Door. Drutt is non-verbal but makes various noises.

The Thing Upstairs (voiced by Willie Rushton) is the impatient, cantankerous, demanding and terrifying master of the castle, who rarely leaves his penthouse room, and consequently is never seen. In most episodes, he orders Berk (in a Cockney accent) to fix things in the castle, prepare meals for him, or sometimes bathe or clean him. His appearance is never revealed, but grotesque hints are dropped: in the 14th episode of the programme's first series, "The Little Thing", a lightning flash illuminates a mass of spongy tentacles and in the same episode, Berk makes a comment about his three eyes; whereas in the 13th episode "The Pain", Berk asks which head contains a toothache, implying multiple heads, and the extracted tooth itself is a fang nearly two-thirds the size of Berk. In a later episode ("Not Very Nice"), Berk loses one of his master's eyes down the trap door, whereafter his master claims to have 'seen' the incident's events through the detached eye; the latter almost as big as Berk. In the episode "The Stupid Thing", it is mentioned that the 'Thing' has three humps on his back; and later, that he possesses wings, which are never shown but can be heard beating.

For a majority of the series, the plot of each episode will revolve around a new monster that emerges from the trapdoor. These monster are often hostile to Berk and his friends, though others seem relatively harmless and simply act as a minor irritation.

The Big Red Thing (voiced by Nick Shipley) is a recurring monster that initially appeared in the first episode "Breakfast Time", in which it emerges from the trapdoor and pursues Berk through the castle, but ultimately flees back down the trapdoor upon viewing its own reflection. The Big Red Thing makes a later appearance in the episodes "Don't Open That Trap Door", along with the final episode of the first season "Bye Bye Berk". Its latest appearance was the final episode of the series "The Big Red Thing", in which it attacks Rogg before exiting the castle. Berk and his colleagues watch as Rogg and The Big Red Thing battle over the horizon, where Rogg apparently dies and The Big Red Thing disappears. The monster soon reappears and roars at the group and the episode ends.

Rogg (voiced by Willie Rushton) is a large gorilla like creature who initially appears in the fourth episode of the first series "Lurkings". Although somewhat unintelligent, he is fairly friendly towards Berk and the other residents of the castle. In the episode "Junk Food", Berk initially dislikes him after Rogg unwittingly gets him into trouble with The Thing Upstairs, the latter mistaking Rogg for a poorly prepared dinner. When confronting the Red Thing during the final episode of the second season "The Big Red Thing", Rogg is pronounced dead as the credits roll, before revealing himself to be alive in the aftermath.

Bubo (voiced by Nick Shipley) is a recurring monster that first appears in the episode "Gourmet's Delight". In the episode, he is initially invisible until he is covered in a yellow substance. Upon catching him, Berk inflates his body through a small hole in the top of his head, before releasing him to soar back down the trapdoor. In the episode "Fester Rancid", Bubo kidnaps Boni and begins repeatedly hitting him with a stick beside a lake, before he is tossed into the water by Berk. Bubo appears for the last time in the episode "Scunge", where he returns to irritate Berk but is ultimately sent back down the trapdoor by Rogg.

The Splund (voiced by Willie Rushton) is a large, round monster capable of teleportation. It was one of the few Trap Door creatures capable of speaking, doing so in a deep, demonic-sounding tone. It appeared in the episode "Don't open that Trap Door", often singing along to the lyrics of the theme song. In the episode "The Splund", it emerged from the Trap Door and began terrorizing Boni and Drutt by teleporting around and threatening to eat them. Berk touched it with the point of a sewing needle, causing it to burst like a balloon and reveal that despite its intimidating manner, it was full of nothing but air. Its voice was edited with a Harmonizer, initially deepened when it spoke, but increasing sporadically when it began laughing.

The television series spawned a video game in the mid-80s called The Trap Door and a sequel called Through The Trap Door. These games were available for the ZX Spectrum, the Amstrad CPC and the Commodore 64.

A board game was also released entitled Berk's Trapdoor Game which involved going around the board while trying to knock one's opponents over by launching one of four dice, each hidden beneath its own trapdoor, in the game board's central catapulting mechanism.

All 40 episodes were released over 4 VHS videotapes in the UK by Channel 5 Video in the 1980s.

In the 1990s, 36 episodes were re-released over 3 videos by Castle Vision (a distribution of Castle Communications plc). The missing four episodes from each of these videos were "Bye Bye Berk". "What a Weirdo", "Nasty Beasty" and "The Big Red Thing"

A rare double video-cassette released in Canada at one point contained every single episode.

All 40 episodes of The Trap Door were released on DVD by Universal Pictures in 2005.

• Roadshow Entertainment (1999)