The film is a direct sequel to The Land That Time Forgot, which initiated the series in 1975. The story follows a rescue expedition, led by Patrick Wayne in search of his friend who had vanished many years before. The expedition lands on Caprona, the same fantastic prehistoric land where dinosaurs and barbarian tribes of men coexist. It is based on the novel The People That Time Forgot (1918) and Out of Time's Abyss (1918) by Edgar Rice Burroughs.

References:
https://en.wikipedia.org/wiki/The_People_That_Time_Forgot_(film)