Jak and Daxter

A young teenager named Jak tries to help his friend Daxter after he is transformed into an weasel-like animal. With the help of Samos the Sage, a master of the mysterious energy called "Eco" created by an ancient race known as the Precursors, the pair learn that they must save their world from the rogue sages Gol and Maia Acheron, who plan to flood the world with "Dark Eco," a mysterious substance which corrupts all it touches.

(Source: Wikipedia)