Stella and her younger brother, Tom, are bored during their summer holiday in London until they discover a hidden, overgrown garden behind their apartment building. Following a mysterious dog, they find a secret lake and a rowboat that transports them 100 years into the past. There, they befriend two children named Harry and Jack, who are searching for a lost family treasure. Together, the children embark on thrilling time-travel adventures, uncovering the past to solve a mystery that connects their families across the centuries.