As Junjie settles into his role within the Shane Gang, he unveils the concept of Slug-Fu, a remarkable bond that allows him to exercise control over his Slugs from a distance. This newfound power faces its ultimate test when all Air and Toxic Slugs in Slugterra inexplicably morph into incurable Ghouls, throwing the team into disarray. Their mission to restore order escalates unexpectedly when they're ambushed by the Shadow Clan, who capture them and transport them to their lair. Within these dark confines, Eli experiences a startling moment when he encounters an Enigmo Slug that bears a holographic message from Will Shane.
The cryptic message uncovers that upon Will’s demise and the looming threat to Slugterra, the Shadow Clan has been tasked with locating Eli and delivering this slug, which carries vital secrets pertaining to the Elementals. According to age-old lore, all slugs trace their lineage back to five fundamental Elemental Slugs, each representing essential forces: Earth, Fire, Water, Wind, and Energy. Should any of these primal slugs fall victim to an unknown corruption, their descendants—including those that have diversified into unique sub-elements like the Toxic Slugs—would also be affected.
With the stakes significantly raised, Eli faces a monumental challenge: to retrieve all five de-ghouled Elemental Slugs and activate them simultaneously, potentially curing the entire slug population of Slugterra. Having witnessed the Wind Slug already succumb to the Ghoul infection, the gang now shifts their focus toward the Water Slug. This journey exposes the devastating environmental repercussions of the Ghouls' influence and unveils a shocking twist—the emergence of a newly formed alliance, incorporating the Darkbane, the reformed Goon, and the mutated Dr. Blakk, who has altered dramatically after a previous confrontation in the perilous Deep Caverns during Eli's past escapades.
As the Water Elemental falls under a shadow, the Shane Gang pivots towards locating the Earth Elemental, their trust in Junjie teetering. A treacherous encounter within the treacherous muck nearly costs them dearly, only to be saved at the last second by the Earth Elemental’s intervention. Meanwhile, the Alliance pursues the Energy Elemental at Quiet Lawn Cavern, only to find themselves thwarted when they arrive and are met by the elusive Red Hook. The Shane Gang follows suit, encountering similar obstacles despite Kord employing an advanced Elemental slug tracker. Upon their arrival, they discover a wounded Red Hook, seemingly doomed until Doc arrives just in time to heal him. An epiphany dawns on Eli as he realizes that Doc is, in fact, the Energy Elemental, guiding them toward the next crucial step in their quest.
With two slugs in hand, the gang decides to take on the final Slug, yet they must first navigate a daunting entrance—a colossal hole that demands only Junjie and Eli to embark on the ascent, relying solely on their Infurnuses. However, disaster looms when the Fire Elemental succumbs to darkness, resulting in both Infurnuses being transformed into Ghouls mid-flight. Quick on his feet, Kord devises a plan, repurposing Mecha-Beasts into jetpacks, providing essential aerial support. As the battle erupts in the heavily ghouled mushroom forest, a shocking revelation occurs: the Goon is estranged from his slinger, revealed to be Eli's long-lost father, Will Shane—a revelation that reverberates through the Shane Gang with profound impact.
As the Earth Elemental grapples with the same affliction plaguing its fellow companions, the Shane Gang dwindles down to a mere handful of Energy Slugs, their last line of defense amidst the chaos. Doc stands resolute, bearing the responsibility of salvaging hope through his scientific expertise to cure the afflicted Elementals and rescue what remains of Slugterra. In an unexpected twist, Dr. Blakk and the Goon join forces with the Shane Gang against their common foes, uniting against the chaos—until a fissure opens in the Wind Elemental, unleashing a terra-portal that pulls Will, Dr. Blakk, and the Goon into the perilous depths of the Deep Caverns.
Navigating the gravity of their dire situation, Eli's resolve is solidified as his allies rally him to unleash the Energy Slugs in a frantic bid to fend off the encroaching darkness and prevent Slugterra's destruction. Confronted with an agonizing choice, Eli ultimately consents to fire the slugs, sealing the portal while simultaneously ensuring the survival of their world. With an unyielding determination to rescue his father, Eli steels himself against the formidable challenges ahead, resolved to conquer any obstacles in his path.
