Tony and Tia Malone are decent, intelligent kids, but have a bad record, partly because they have grown up in a slum, and partly because Tia ignores rules and common sense whenever she sees an animal in distress (she once broke into a store to free a trapped kitten, so she has a record).
They conceal the fact that they have Psychic Powers, and that Tia can talk (her voice is pitched beyond the range of most people's ears, other than Tony's).
They do not know where they originally came from; they have lived with Granny Malone, who was struck and killed by a car the day before the story opens. Because of their record, they are then packed off to an Orphanage of Fear, and begin trying to make a serious effort to remember their past. Their only clue other than their own memories is Tia's "star box" - a leather box on a strap with a curious double star design on the lid, which she has had as long as she can remember and which she always carries (since it contains the pencils and paper she uses to communicate with people other than Tony). A chance encounter with an elderly nun gives them another clue - she recognizes the design on the box as something she once saw on the letterhead of a letter, looking for children with "unusual abilities". She cannot quite remember who wrote the letter, but from what she does remember, the children eventually come up with the name "Castaway".
Tia later finds a map to Witch Mountain - which is in the Blue Ridge Mountains - and a lot of cash concealed beneath a false bottom in her "star box".
Matters come to a head when a man comes to the Orphanage of Fear claiming to be their uncle - Lucas Deranian, whom Tia recognizes as the man who originally left them with Granny Malone, and who she dislikes and distrusts on sight. The kids run away after being told that he is now their Illegal Guardian, heading for Witch Mountain. When they finally reach their destination it's discovered that they were originally from an alien planet. They had to come to earth to survive a planet-wide disaster, so the grownups built a colony on Witch Mountain.
Tropes seen in this story and its adaptations include:
- Alien Among Us: What the Castaways turn out to be.
- Alternate Continuity: The Beyond Witch Mountain film ignored the first sequel.
- Bald of Evil/Beard of Evil: Bolt and his henchmen have one or the other.
- Beary Friendly: The kids use a bear to help them escape from Bolt's henchmen.
- Half-Identical Twins: Tia and Tony in the movie only. In the book, they are about a year or two apart with Tony the elder.
- Honorary Uncle: Uncle Bene was not actually the twins' actual uncle.
- Instant Sedation: Used in the second movie to take down Tony. Later, Gannon uses an oxygen tent to keep Tia indefinitely unconscious in "comatose neutralization". Slightly averted in that she's noticeably groggy upon being released rather than recovering instantly.
- Kind Hearted Cat Lover: Tia loves her adopted cat Winky.
- Magic Music: Tony needs his harmonica to use his powers for more complicated tasks. It is implied that his powers are weaker than Tia's because of this. In the puppet show seen they actually talk about this.
- No Good Deed Goes Unpunished: The twins catch Bolt's eye after they save one of his men.
- Orphanage of Fear: Subverted in the film. In the book, Hackett House was more of a Juvenile detention facility or halfway house for minors.
- Parental Abandonment: The kids' only guardian was killed in a car accident the day before the story opens.
- Photographic Memory: Tia especially has this, remembering events perfectly as far back as before she was two years old. Due to the terrible events that took place, she had Trauma-Induced Amnesia where she was scared to remember the vital memories of who they were and how they got there.
- Pink Girl, Blue Boy: In the movie, Tia's bed is decorated pink.
- Psychic Powers: Tony can visualize far away places and see what is happening there. He and Tia both have telekinesis (although hers is mostly used for opening locks).
- Speaks Fluent Animal: Tia tells Tony that she doesn't so much speak animal language but understand them, which is actually similar to being able to speak to them. Tony also can play music that mingles with the sounds of the wild which attracts animals to him. If he stops playing, the magic ends.
- The Speechless: In the book, Tia and most of their race never learned how to talk as they can communicate telepathically. Tony was notable for being able to speak.
- Averted in the movies.
- Super Senses: Both Tia and Tony had super hearing. Tia got in trouble once for hearing a cat in distress in the back of a grocery store.
- Both of them thought of Tia's way of communicating as supersonic speech that couldn't be heard by anyone who didn't have their super hearing.
- Trauma-Induced Amnesia: Although Tia has a near-perfect memory, she cannot remember anything prior to their arrival at Granny Malone's.
- Unusually Uninteresting Sight: In the movie, anyway, the kids at the orphanage seem to get over the shock of seeing psychic powers very quickly.
- Witch Hunt: Understood when Tony causes a rain coat and broom to dance around and frighten the local NC sheriff. Also when Tia removed the locks from the bears' cage, it spooked the locals.