Aging geologist Angus McKinnon finds a baby Asian yeti, Lug, and takes him back to Oregon to rear. Lug is quite intelligent. He learns simple speech, several games, enjoys music, but becomes dangerous when mature.
Angus seeks help with Lug by flying him to a Zoology convention. The plane is blown off course into Mexico and runs out of fuel. He comes down inside a cloudy, wet, garden-like volcanic upthrust and is marooned.
Also on the plane is a tree-climbing kangaroo from Australia sent to Angus by an old colleague, Henry Stoddard. He wants Angus to take the strange animal to the convention hoping it will be identified as a new species. This creature is important in the story.
Jerry Worthington, a young, impetuous geologist, sees the unusual volcanic formation from his company's helicopter. His insatiable curiosity compels him to go there alone. He Jeeps across desert country, climbs the protrusion and meets a nasty surprise.
Lug, to Jerry's astonishment, knows and loves the game of chess. They play and Jerry realizes he must lose or die under Lug's hideous teeth and hands. Jerry learns that Lug killed and devoured three others who came to the tower. The revolting animal gloats about eating Angus' body after he died.
McKinnon wrote a journal about Lug's development that Jerry reads as the story progresses.
Maria Hernandez, Jerry's fiancée, senses he is in desperate trouble and goes to the cloud tower to help him. Does she succeed?