Four boys find a secret cave inhabited by dinosaurs from the exhibit at the New York Museum of Natural History.

Filmed on location in England and Morocco, the story concerns 13th-century Saxon nobleman Walter of Gurnie, who, after sparking an unsuccessful rebellion against the Norman conquerors of his homeland, sets out to seek his fortune in the Far East.

Duke Cesare Borgia attempts to marry his sister off to the heir of Ferrers, which would give him control over a wider part of Italy. He employs a soldier, Andrea Orsini (his sister's lover and an unscrupulous man) on this quest. En route, Orisini learns a more honorable way of life and turns against the Borgias and leads a revolt against them.

Captain Lee and his crew find themselves in grave danger when they crash land on a planet seemingly similar to Earth, but millions of years in the past and filled with deadly dinosaurs.