The Mellop family, Bill Mellop, and his children, Samantha, Michael, and Jason will be spending their first Christmas alone since their mother died. Also the plant nursery run by Bill and his sister is under threat from a difficult and mean new neighbour, Berrie Dump. Samantha sitting with her dog Ajax, notices a mysterious falling star that comes in through their window, and crashes into a crystal punchbowl. The star refracts into seven smaller rainbow coloured stars, one hitting Ajax's new Christmas collar, and another Samantha's frilly apron. Strange things begin to happen. Samantha can hear Ajax talking, but when she asks Jason to listen he hears nothing. When Jason jokingly dons Samantha's apron he too hears Ajax talking. The frilly apron must be magic.
The dog collar and the frilly apron enable the kids to communicate by telepathy. Michael's favourite T-Shirt makes him disappear and he sneaks into Dump's office. He hides the dog collar in Dump's hat to tune in to his thoughts and foil his next move. Michael tries to impress the beautiful Harmony with the T-shirt of invisibility but she tells her father Bernie Dump!
Dump orders Harmony to use her friendship with Michael to seize the apron. A strange Kombi van makes its way towards Earth with a childlike man named Alpha-Ralph 416 and an intelligent girl named Commander Gamma Jane 378. The boys try to recover the collar from Dump's home while Ralph and Jane have an unpleasant introduction to earth. Aunt Jocelyn suddenly speaks a mixture of English and German. Jane tells Samantha she knows the family has their leader and that she and Ralph are there to take him back to Moon.
Jane and Ralph have been sent by Her Royal Babiness the Grand Baby, to find the Moon's new ruler. Jane is in charge because she's younger it seems everything on the Moon is in reverse, the ruler is a Grand Baby and the kids boss the grown-ups around. There are still four items in the house with special powers of linguistic ability, flight, time travel and speed and they must be found, and placed with the apron, collar and T-Shirt so the star can be reformed and the Chosen One found. Meanwhile Harmony is showing her father the T-shirt of Invisibility.
The kids believe it was one of Aunt Jocelyn's bras which gave her the linguistic powers but try them on without success. Harmony washes the T-shirt of Invisibility in a bucket and when Dump throws the rest of the water at a barking dog it disappears. He bottles the water and sets out in his truck with a new sign, Royal Dump Pest Exterminators. Jason, wearing his baseball cap, when his feet suddenly leave the ground and he gains both altitude and speed. Bill is flabbergasted when he sees his son fall out of a tree, only to claim he didn't climb the tree but flew into it!
The Royal Dump Pest Exterminator company is making a fortune, making ants, spiders, and even rats disappear and Dump buys a new mansion. Harmony wears the baseball cap and floats away with it. The family retrieves the baseball cap from Dump, who is horrified, as the dog becomes visible again. The Grand Baby has only six or seven days to live, so the rest of the powers must be found so the new leader can be identified. The bathroom toilet bowl must give linguistic ability!
Pests sprayed with water from the T-shirt of Invisibility are re-appearing and Dump is besieged with calls from furious customers. Bill orders Michael to the nursery, leaving the others desperately trying to find the power of the toilet bowl—to no avail. Dump realises the power involves something Aunt Jocelyn uses in the bathroom and works out a plan to get to know her better.
The Grand Baby has only days to live, but attempts to reach Jane by telephone are hampered by Dump. Bill orders the kids to vacuum and Ralph is watching a dinosaur show on TV when his foot hits the power button and both he and the vacuum disappear. Ralph is nowhere around—instead he's clutching the vacuum cleaner and wandering in a primeval swamp of 300 million years ago, inhabited by giant dinosaurs.
Dump sees a pair of slippers careering around the Mellop's hall. He tries them and shoots away at great speed to the local oval where he charges onlookers for the ride of a lifetime. The kids are watching a pirate program called Swashbuckle's Fortune on TV, when Ralph reappears. Seeing the mess Ralph has made, they are all touching the vacuum cleaner when the power is switched on. They vanish and reappear on a beach—where they face a murderous band of pirates, led by the frightening Iron Peg.
The kids encounter a band of battling pirates via the Vacuum Cleaner of Time Travel but rocket back to the present—complete with Peg's leg. They estimate they now have four of the six powers unaware that Dump has the Slippers of Speed, which have since slowed down. Michael and Harmony search the Kombi for something to help Dump make another fortune but hit some buttons and find themselves hurtling into space.
The Kombi lands on the moon's surface and the Grand Baby thinks Jane and Ralph have returned with her successor. When Michael hits the Earth button, the Kombi takes off, and the Grand Baby realises Jane and Ralph can't have been inside. Jason tries to get Aunt Jocelyn back into the bathroom where they suspect the power of linguistic ability is hidden. Dump tries to steal the Kombi and Jane disables it by removing an item, which looks like a metallic golf ball but is actually a miniature nuclear reactor.
Dump learns of the cold fusion reactor and asks the kids to swap the reactor and the Kombi for the Slippers of Speed. Jane gives in to Dump's demands, but reactor has disappeared. Aunt Jocelyn has mistaken it for a golf ball and it will destroy the entire planet if struck. Ralph retrieves the reactor and returns it to the Kombi, while Dump sets up a new business venture—the full horror of which is revealed as the Kombi van dumps tonnes of garbage on the pristine surface of the moon.
The Grand Baby believes Jane and Ralph have returned with the new leader, but the Moonlings' excitement turns to confusion when the Kombi dumps a load of strange objects and departs. Michael becomes suspicious about the material Dump is dumping in space. The kids tell their father the truth about what has been going on and Bill tries to call Jane's parents on the number she has given him. Aunt Jocelyn emerges from the bathroom holding a toothbrush and speaking a foreign language. Does the toothbrush have the Power of Linguistic Ability?
Samantha wears the apron and slippers and holds the baseball cap, t-shirt, vacuum cleaner, dog collar and toothbrush, as Jane holds the punch bowl over her head, to discover if she is the Chosen One, but one of the items is still wrong. The kids decide the toothpaste had the power but Aunt Jocelyn has tossed the old tube out with the rubbish. When they find it in the garbage Samantha scrapes out a sliver—and begins to talk in a foreign language.
Samantha realises she is not the Chosen One and thinks it is Jason. Jason is draped with the items of power, but scarcely a spark is raised. Michael refuses to be tested and a crazy chase follows. Michael and Harmony investigate the nature of the garbage being left on the Moon and learn Dump is dumping disposable nappies on Moon. The kids dress Michael in the items of power while he sleeps but he is also not destined to be Moon's new leader and they decide it must be Bill Mellop. Samantha is distressed because having lost her Mum, she now has to face losing her Dad as well.
The Grand Baby is gone and the need to test Bill as her successor is vital but the Miraculous Items go missing. Michael is suspected of the theft but Dump arrives at a party draped in the items. When he realises what the outfit could mean, he plugs in the vacuum cleaner and reaches desperately for the full punch bowl. The kids are shocked to see him with the powers, but more so when they realise Dump was dumping garbage at the house the night the star struck—and chances are he could be the new Grand Baby!
Samantha realises she will not lose her dad if Dump becomes the Grand Baby so the kids decide to help, but efforts-to empty the punch bowl fail because Ralph has added the tablet which keeps it topped to the brim. As the party guests are pressured to drink litres of punch, Michael discovers Dump is peddling nuclear waste to the moon. Samantha convinces Bill to try on the miraculous items, and the Moon's Chief Minion gives her the number Grand Baby provided before she died—22,121,986. Now it's vital the kids find out what it means!
Jane, Jason and Samantha search for an equation, which will explain the Grand Baby's list of figures. Dump reassures Harmony the waste on Moon is safe but millions of miles away three Moon Drones have fallen ill after playing with one of the barrels.
The Chief Minion will free Michael but Harmony can only go free if Dump agrees to remove all waste from the moon. The mystery numbers are the birthdate of someone born in the Mellop home, but only Michael's late mother was born in the house the Minions ignore further pleas for release. Dump realising the full portent of his misdoings, and what he must do to free his daughter, brings a load back from Moon and cancels his waste contracts. But when the Kombi runs out of fuel he is unable to return for Harmony. Her only hope is to zip down a space/time corridor, which will open when the star sends the Chosen One back to Moon.
Michael grabs the vacuum cleaner, hits the power switch and disappears. When Michael reappears with an idea on who is the new Grand Baby. The kids re-assemble the magic powers and place the punch bowl over the head of their last hope. As someone disappears from the Mellops, destined to be the new Grand Baby, Harmony arrives home. Michael finds a Christmas card from his mother's and reads her last loving message—he too now believes in magic.