In The Lion's Den takes place in an alternate Earth where anthropomorphic animals called “critters”, coexist with the primitive “Giants”. The Critters live in a land of their own, forming social groups and often gangs due to conflicts between one kin to another. Many of the Critters also venture out into The Land of the Giants, reasons for this ranging from opportunism, to feeling isolated from their own kind, or simply out of curiosity. Subsequently, many end up forming special bonds with The Giants.

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The series revolve around five critter protagonists who somehow become separated from their Giant companion and get thrown back into their native land, The Fringes. The home that they once knew is corrupted by powerful beasts called "Therians" in a reign of utter tyranny. With the aid of other critters encountered along their journey, the protagonist must conquer the beast, and find their way back home.