It's been awhile since I talked about the first two books in this series, so you may want to refresh your memory.
The third book opens with four men about to crash in a plane. They are, of course, the Challengers of the Unknown. Meanwhile, Hal Jordan has just accepted his job at Ferris Aircraft. The Suicide Squad battles a giant Pterodactyl, which kills Rick (the leader) before it dies as well. At his funeral, one of the women remarks that she never saw anything like that before in her life, but a WWII vet mentions seeing something like it on an island in the South Pacific. This is the first foreshadowing of the finale of the story.
Hal Jordan discovers that life at Ferris is nowhere near as exciting as he thought it would be, as he is subjected to a seemingly endless series of what seem to be useless tests. Then, just as he's about to quit Carol reveals:
They're planning a trip to Mars, with Hal as one of the crew. At the same time, Batman prods John Jones to obtain a book that was entered into the evidence locker when the two of them broke up a cult a year and a half earlier. When Jones does, he discovers that it tells a story of a Viking Prince who was cast adrift by his crew and landed on a mysterious island, where giant creatures (dinosaurs) lived. He barely managed to escape. At the end of the book is a description of a terrible force which dwarfs the planets of the solar system. Jones is shocked to realize that the force is already here.
Comments: The story is starting to take shape; we see how Cooke has taken the dinosaurs and monsters of the Silver Age and explained that they all had some common origin.