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Mark and Lisa need to help their Uncle George (the current guardian) stop an ancient box from being opened. The box is a doorway to a slave world.

George, Mark and Lisa begin the search for the three keys to the box. Lisa is awakened during the night by people who think her uncle has the first key.

Lisa and Mark are searching for the second key; when they find it, the Watchers try to kidnap them.

Having escaped from the attempted kidnap by the Watchers, the twins join up their Uncle George to locate the third and final key at the Roman baths.

Now having all three keys, George and the twins persuade Professor Waters let them in to see the box.

George breaks into the Janus Foundation to try to steal the box back, but then Mark and Lisa realise it is, in fact, a trap.

The gateway to the world of Heritron is now opened. George and the twins escape from the Janus Foundation, but are then betrayed.

Professor Waters retrieves the Box back from the Watchers' Headquarters, and George goes through the portal to lock the doorway into Heritron forever. The Catchers are waiting for him on the other side.

George is held against his will on Heritron. Anna tells Mark and Lisa that they must rescue him.

The twins find George imprisoned in a crystal cell. They manage to break him free, but can they escape from Heritron and get back home?