The doorway: Other world.

Three teenagers -- Wendy, Brainy, and Thumper -- are drawn into another world when they follow directions in an old book that they found in a reputed haunted house.

They are met in this world by a little person warning them that they need to follow him. He leads them into an underground town run by a very tall man named Kor. Kor convinces them to accept a mission to destroy a meteor that has hit their planet and is now poisoning it.

The teenagers cross a landscape inhabited by giant snake...