Subtitled: "A fantasy game providing equal time for the monsters."
Play the monsters waiting in the dungeon for parties of adventuring humans, elves, and dwarves to come along! Win fame and glory and loot, loot, loot! This book covers the flip side of the average dungeon-delving adventure.
(The first edition of Monsters! Monsters!, published by Metagaming in 1976, was not fully compatible with T&T. The Monsters! Monsters! (2nd Edition) is fully compatible with T&T 5th edition, though from an entirely different perspective.)