THE STORY REVOLVES AROUND FIVE HUMANS: JAKE, MARCO, CASSIE, RACHEL AND TOBIAS, AND ONE ALIEN, AXIMILI-ESGARROUTH-ISTHILL (NICKNAMED AX), WHO OBTAIN THE ABILITY TO TRANSFORM INTO ANY ANIMAL THEY TOUCH. NAMING THEMSELVES "ANIMORPHS" (A PORTMANTEAU OF "ANIMAL MORPHERS"),THEY USE THEIR ABILITY TO BATTLE A SECRET ALIEN INFILTRATION OF EARTH BY A PARASITIC RACE OF ALIENS RESEMBLING LARGE SLUGS CALLED yeerkS, THAT CAN TAKE ANY LIVING CREATURES AS A HOST BY ENTERING AND MERGING WITH THEIR BRAIN THROUGH THE EAR CANAL. THE ANIMORPHS FIGHT AS A GUERILLA FORCE AGAINST THE yeerkS WHO ARE LED BY VISSER THREE.
THROUGHOUT THE SERIES, THE ANIMORPHS CAREFULLY PROTECT THEIR IDENTITIES; THE yeerkS ASSUME THAT THE ANIMORPHS ARE A STRIKE FORCE SENT BY THE ANDALITES, THE ALIEN RACE TO WHICH AX BELONGS THAT CREATED THE TRANSFORMATION TECHNOLOGY, TO PREVENT THEM FROM CONQUERING EARTH. TO PROTECT THEIR FAMILIES FROM yeerkS REPRISALS, THE ANIMORPHS MAINTAIN THIS FAÇADE.