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 is accompanied by 5 new slugisodes and a mobile game called "Slug It Out: Shadow Wars".
- 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.
- It is unknown how Pieper found her way to The Surface, but Tad says he saved her from a cat.
- According to Tad, Pieper had been helping in mapping the underground tunnels to SlugTerra for years.
- This is the first time that Eli mentions his mother in the series.
- Eli looks unfamiliar with fist bumps when Tad tries to give him a fist bump. Though fist bumps are a way of greeting in Slugterra since Kord, Pronto and Junjie have been seen doing it, notably in "The Unbeatable Master", "Slugterra: Return of the Elementals" and "Stuff of Legend".
- Eli may be more accepting in high-fives and handshakes as greetings compare with fist bumps.
- Despite the protoform of the Diggrix is first seen in "Slugterra: Slug Fu Showdown", its velocimorph is first seen in this movie.
- In one of the scenes that the slugs are walking into the Citadel, a Megamorph Blastipede is seen.
- Pronto is the only member of the Shane Gang to be suspicious of Tad since the beginning due to Tad's smell.
- It remains unknown how Tad finds out that he is the son of Dr. Blakk, who resides in Slugterra.
- Eli officially turns 16 as revealed at the end of the movie.
- This episode takes place on Eli's Birthday, like the first episode in the series.
- When Tad and Pieper take a drop to SlugTerra, they're seen using the same route as Eli and Burpy in "The World Beneath Our Feet Part 1".
- The Gateways used in "Slugterra: Eastern Caverns" is seen again.
- The King of Sling tells the Shane Gang that the Shadow Clan don't like rockabilly music, which was what he asked them after the Shane Gang left the 99 Caverns in "Slugterra: Eastern Caverns".
- Pronto being the only Shane Gang member to be suspicious of Tad is the same as Kord being the only one suspicious about Twist in "The New Kid Part 1" and "Part 2".
- The blaster given to Tad is supposed the old blaster that Eli used to use until "The Slugout".
- The Scrap Force once again calls themselves "The Tough Guys", which is the original name they used in "The Hard Part".
- The Citadel is seen again and looks to have been left in the same condition as last seen in "Light as Day".
- Bob and Blast returns since "Slugball".
- The people in Quiet Lawn Cavern are now able to make loud noises due to the disappearance of the Grenuke slugs, in which they previously expressed their concerns throughout several episodes.
- The interior of Blakk's old stronghold are exactly as how they were left at the end of "Light as Day".
- Gattlers once again make a return to the series since their last appearance in "Slugterra: Return of the Elementals".
- Dr. Blakk once asked Eli about "The Burning World" (The Surface) in "Back to Blakk". However, it is unsure whether Dr. Blakk already knew about The Surface and about his son, Tad, who had been living on The Surface.
- Tad references the defeat(s) of his father, Dr. Blakk, against Eli.
- The scene when Trixie tries to high-five the Shadow Clan and only gets the other's confusion in return is the same scene in "The World Beneath Our Feet Part 2" when Eli tried to high-five Kord, which confused him.
- The chasm created by Terraportal in "Light as Day" returns and just like his father, Dr. Blakk, Tad falls into the hole.
- Pronto says that they are getting good at throwing people down holes, hinting the events of "Light as Day", "Slugterra: Return of the Elementals" and "The Return of the Eastern Champion".
- The message from an Enigmo slug is used again since "Slugterra: Return of the Elementals".
- The scene in The Deep Caverns is similar to the one seen in "What Lies Beneath".
- In the scene when Tad commands the Scrap Force 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.