When Fatty and his sister George decide to surprise their scientist father by bringing him lunch at work, they, along with their friend Izzy, look on as he disappears from his lab in a flash of light. The kids grab the crystal that controls their father's time machine and flee from the evil woman their father was working for. The kids quickly discover the crystal's ability to freeze time and seek out the help of their aunt Kath and grandfather Bert.
With their father gone, Fatty and George move in with Kath and Bert. The crystal takes control of Fatty's dreams making him scetch out the plans for his father's time-travel module. The mess he generates while doing so gets the kids in trouble with Bert but the time-stopping power of the crystal allows them to set things right. Meanwhile, Izzy is abducted by Nancy and her brother in an attempt to learn of the crystal's whereabouts. The kids find a hideout where they can keep the plans and the crystal safe.
Fatty and George get the key to their grandfather's old boatshed and together with Izzy, clean up the place so it can serve as their new hideout where they can work on building a new time-machine from their father's plans. An encounter with a pair of bank robbers reveals the crystal's ability to turn back time. Their reward for catching the crooks earns the kids a reward which they use to purchase parts for the time-machine.
Izzy convinces Fatty and George to use the crystal during a long-distance race to settle a score against Slasher. Seeing the effect the loss has on Slasher, Fatty decides to use the crystal again during an exam to help restore Slasher's self respect. When that backfires, Fatty joins the football team to finally set things right — by doing everything wrong.
The kids finish construction of a secret entrance to the boatshed via a storm drain. Fatty decides the best place to hide the crystal is in the graveyard. Nancy, disguised as a woman from the Social Welfare Department, picks up Fatty and George demanding to know the whereabout of the crystal. Izzy unknowingly leads Phil to the crystal but Phil inadvertantly activates it allowing Izzy to get it back and rescue his friends. The trio scavange parts for the time-machine from the scrapyard owner by Slasher's father.
Having lost his hat at the scrapyard, Fatty is blamed for a subsequent robbery and is forced to reveal the location of the hideout to Slasher's gang. Using the power of the crystal to turn back time, the kids escape from Slasher, solve the robbery, and see that the stolen car parts are returned. The kids find a bomb burried in a vacant lot.
In need of a metal cone for the time-machine, the kids find just what they need when they come across a bomb buried in an empty lot. The disappearance of the WWII era bomb alarms the local residents and Slasher and his mates, the number one suspects having been seen in the area, direct the police to Fatty, George and Izzy. The Civil Defense Force trace the bomb to Fatty, George and Izzy but the trio manage to avoid capture and return to the hideout. Unable to return to their homes, the kids realise they are officially "on the run."
Fatty realises their only hope of saving their father is to focus on finishing the time-machine, stealing whatever remaining parts they need. Their last requirement is a "transporting chamber", for which they borrow Bert's shower stall — while he's using it. Nancy and Phil try to discover why the kids are immune to the crystal's power to obtain the same benefits for themselves. Slasher and his mates join the search for Fatty and George in hopes of a possible reward. While time is frozen, the kids are surprised by an approaching car with Nancy and Phil inside.
The kids are puzzled by Nancy and Phil's sudden immunity to being frozen when the crystal stops time, which is reason enough for Fatty to not want to use the crystal again. George refuses to allow their mouse to be the test subject for the time-machine but no suitable alternative can be found. The kids return to Kath's for a hot meal. Kath has come to believe the kids story about the time-machine and even manages to convince Bert that they have been responsible for all the odd unexplained incidences around the house. Slasher and his mates catch up with the trio first. While Jonesey goes to call the police, Fatty tells Slasher the truth, earning new allies who help them avoid capture.The united gangs return to the hidout but Nancy and Phil are close behind.
Bert convinces the kids to let him help and convinces them to go to the police to have them deal with Nancy and Phil. Seeing them leave hideout empty, Nancy and Bert enter and detroy the time-machine in during their search for the crystal. Phil and Nancy are still inside when the kids return but the kids escape, grabbing the keys to their father's lab along the way. As all parties converge of the lab, Fatty gets his father's time-machine working and successfully rescues him. Nancy and Phil arrive and force everyone into the time-machine but when it is activated, it is Nancy and Phil, standing outside the machine, that disappear — along with the crystal. Despite no longer having the crystal, Fatty, George and Izzy decide to rebuild their time-machine in hopes that one day the crystal will find its way back to them.
