While on holiday, teens Simon Randall (Spencer Banks) and Liz Skinner (Cheryl Burfield) stumble across an invisible barrier that lets them travel through time in this 1970s British sci-fi series. The discovery sets them off a series of adventures leading to a World War II naval station, a futuristic Antarctica and a top secret research center. Guiding the youngsters on their incredible journey is the mysterious Charles Traynor (Denis Quilley).