Synopsis: From the first page of the Sentinels from Space side of the Ace Double: "They sent one man to block an interplanetary war! "We have been at war for eighteen months and have only just discovered the fact! Venus and Mars are attacking us - and we can't do a thing about it!" That's what the World Council told David Raven when they gave that space pilot the most dangerous mission of his career. Earth's rebellious colonies on Mars and Venus were out to win their independence by fair means or foul. Being still technically citizens of Earth, they could send their sabotage squads down without legal interference to spread havoc and wage secret warfare. And those traitorous colonists were talented beyond all normal abilities. Some were super-hypnotists, some could read thoughts, some could start fires by merely pointing a finger, and some could change their features to match anyone's. Single-handed, David Raven was supposed to outwit them all and restore peace. How Raven handled these dread foes, how he personally invaded Venus, just who and what the amazing SENTINELS FROM SPACE were, is an epic science-fiction adventure." From the first page of the Ultimate Invader side of the Ace Double: "Great science-fiction from the four corners of time! Here's an exciting new anthology in which four famous writers present breath-taking adventures that space the whole of time and space. Never before published in book form, these stories have been specially selected to combine all the elements of first-rate science-fiction entertainment. Eric Frank Russell's complete short novel gives a vision of the farthest future and of a conflict that involves two galaxies. Malcolm Jameson's unusual novelette deals realistically with the problem of the near future's conquest of space and the impact of interplanetary life. Murray Leinster takes us face to face with a terrible danger from humanity's near past. Frank Belknap Long spans the entire course of human history to throw a boomerang into the very dawn of life. This startling new collection is an ACE Original, edited for ACE BOOKS by Donald A. Wollheim, well-known anthologist and science-fiction expert."