A new member has joined Eli and the Shane Gang! Junjie, once the protector of the Eastern Caverns, is a master of the slugslinging art of Slug Fu! But even with the power of five slingers, the Shane Gang find themselves in over their heads as they race across The 99 Caverns in search of the Legendary Elemental Slugs. The five Elementals are ancient slugs of great power, and the forbearers of all slugs found in SlugTerra today. In the wrong hands, they could bring Slugterra to the brink of destruction. So when an evil alliance starts hunting down the Elementals, Eli and his friends — old and new — take off in pursuit of the greatest threat their world has ever faced!

To impress a beautiful girl, Harold Kelp, the grandson of professor Julius Kelp, who created a potion to transform his personality, gets his hands on his grandfather's secret elixir and unleashes his destructive alter ego, Jack.