As the Shane Gang return to the 99 caverns from their Eastern adventure, a dispute erupts between them and the Shadow Clan, threatening to shatter the hard-won peace in Slugterra. Eli and the Gang have their hands full dealing with the antics of the usual thugs, plus growing hostility from their former allies. So when Tad and his slug Pieper—who have just arrived from the Surface—lend a hand with Pieper’s amazing powers, Eli welcomes a new team member. But soon this newcomer casts his own shadow on the chaos.
IMPORTANT NOTICE: For TV purposes the movie is divided into 4x22min episodes.
Part 1 - Back on the Shane Gang
As the Shane Gang returns to the 99 caverns from their Eastern adventure, they find Gateway Cavern overrun with slugs. In the Shane Gang's absence, the Shadow Clan took a different approach to protecting the caverns, putting slugs before the needs of people and it's up to Eli and the gang to restore balance.
Part 2 - The New Boss
The Shane Gang takes Tad under their wing, training him to become a great slugslinger with Eli's old blaster. Their training is put to the test when the Gang brings Tad to answer a distress call that leads to a showdown with the Scrap Force and the Shadow Clan.
Part 3 - Out of the Shadows
Tad is missing after the slug-out with the Shadow Clan, so the Gang sets off without him to investigate reports of slugs disappearing. The mystery deepens as the trail of slugs leads straight to Dr. Blakk's old stronghold!
Part 4 - Ghouls and Monsters
Tad's hypnotic slug Pieper has gone megamorph, his gang of supporters all have Gattler blasters and Tad's control over his new ghouls is stronger than Dr. Blakk's ever was. The Shane Gang are under non-stop ghoul fire as they struggle to put a stop to Tad's plans and the mass ghouling of slugs once and for all.
- Eli Shane
- Beatrice Sting
- Kord Zane
- King of Sling
- Blast Vanderhuge (Cameo)
- Bob Johnson (Cameo)
- Shadow Clan
- The Scrap Force
- Locke and Lode
- Dr. Blakk (Mentioned only)
- Will Shane (Cameo)
- Jimmo Shane (Possible mentioned only)
- The Surface (Cameo)
- The 99 Caverns
- The Eastern Caverns (Mentioned only)
- The Deep Caverns (Cameo)
- Sam Vincent - Eli Shane
- Lee Tockar - Pronto, Lode
- Shannon Chan-Kent - Trixie
- Andrew Francis - Kord, Mongo
- Adrian Petriw - Tad
- Brian Drummond - Shadow Clan, Boss Ember, Blast Vanderhuge
- Trevor Devall - King of Sling
- Ian James Corlett - Straggus
- Dale Wolfe - Bob Johnson
- This movie will be accompanied by 5 new slugisodes and possibly a mobile game called "Slug It Out: Shadow Wars", but no further information on its current status has been released.
- Tad and Pieper travel down to Slugterra using The Drop that was used by The Shanes.
- It is unknown how the Shane Gang activates The Gateways used back in Slugterra: Eastern Caverns to return to The 99 Caverns as four out of the five Elemental Slugs are left in The Eastern Caverns to be Guardian Slugs.
- In one of the scenes that the slugs are walking into the Citadel, a Megamorph Blastipede is seen.
- Eli officially turns 16 as revealed in the end of the movie.
- Pronto is the only member of the Shane Gang to be suspicious of Tad since the beginning due to Tad's smell.
- This episode takes place on Eli's Birthday, like the first episode in the series.
- It is possible this is the series finale.
- In the scene when Tad commands his supporters to shoot their Ghoul Slugs, Pieper is seen in her Megamorph Protoform despite she only achieves the upgrade later.
- Before using the Double Barrel, Eli loads up his Grenuke and Kord's Rammstone. But when he fires them to perform a Fusion Shot, the Grenuke is a Hop Rock.