Within the buried recesses of the human brain there lurk the ravening beast instincts of a million years ago. Thousands of years of growing intelligence and social culture has buried this deadly strain - until alien science learned how to release the killer impulse of the Earth-born psyche. The aliens were willing to sell their know-how to the highest bidder in exchange for an alliance against Terra. They had many willing to bargain with them.

This was a serious problem for FATE's Free Acting Terran Envoys - and their top man, Cap Kennedy, was assigned to it. But Cap realized rapidly that the ENEMY WITHIN THE SKULL lurked inside his own mind too - and that canceling the plot might mean first releasing the demon thereby turning himself and his friends into monsters that would no longer be men...