Peter, Janet, Jack, Colin, George, Pam, Barbara, and Scamper the dog are desperate for a new mystery, so they decide to practice their detective skills by trailing strangers around the neighborhood. But Jack's mischievous sister, Susie, and her friend Binkie decide to ruin the fun. They set up a cruel practical joke by laying a fake trail for the Secret Seven to follow, leading them straight to the creepy, abandoned ruins of Tigger's Barn in the dead of night.

But Susie's harmless joke quickly takes a terrifying turn! When the Seven arrive at the ruined barn in the dark, they accidentally stumble upon a very real, very dangerous gang of criminals using it as a secret hideout. Overhearing a sinister, high-stakes plot involving a nearby railway yard and a massive robbery, the children realize they are the only ones who can stop it. Plunged into a blinding, freezing fog, the Secret Seven must use all their trailing skills for real to track the ruthless thieves through the dangerous, echoing railway sidings before the criminals make their getaway!