A condensed-matter physics experiment in a university physics laboratory produces an unexpected breakthrough, when the apparatus begins swapping normal matter with "shadow matter". Industrial espionage goes awry and young physicist David Harrison and two small children find themselves inside a giant tree in an alternate Earth populated by strange, wonderful, and dangerous six-legged wildlife. David and the children must find a way back, while dealing with the local fauna and peeking and poking at the Earth they left to thwart the agents who caused their problems.