"The Battle for Subterranea-Tania" is the thirteenth and final episode of Billy Dilley's Super-Duper Subterranean Summer.
As Billy and friends prepare to leave Subterranea-Tania by throwing a farewell party, festivities are interrupted by an alien invasion.
As Zeke and Marsha pack up their essentials for the Cheeserator, Billy begins powering a crystal that he found to take them home as summer vacation has ended. He states that he needs to be delicate on how he does it otherwise he could overcharge the crystal. Zeke and Marsha are ready to head home, but Billy suddenly realizes that they have not informed their friends in Subterranea-Tania. He suggests throwing a going away party which the other two agree to. Billy is overcome with sadness, but upon entering the party is surprised to see everyone he knows are having a good time. Misunderstanding the situation, Billy becomes furious and insults everyone. He then heads out to charge the crystal, but in his emotional state ends up overpowering it.
In space, a race of alien robots pick up the crystal's power and head straight down to Subterranea-Tania where they imprison everyone in a giant electric fence to get the crystal. Everyone realizes that this is all Billy's fault and are further angered when he accidentally suggests that they dig for more crystals. When Billy tries to qualm the situation all the Subterraneans tell Billy that he is not their friend because of how obnoxious and annoying he is. While having paprika added to his food, one of the robots sneezes and breaks a hole in the gate. Billy, Zeke and Marsha escape, but Billy decides to go back and rescue everyone despite what they told him with Marsha and Zeke agreeing as they need the crystal.
Billy, Zeke and Marsha make it to the exterior of the cage and use a discarded robot as a disguise to free everyone. They are successful, but as they reach the top of the hill, they look down and see that the rest of Subterranea-Tania encased in similar electrified fences. Everyone agrees that they should just run and hide, but Billy, as well Gorkager, agree that they should stand and fight. With newfound enthusiasm, everyone charges for battle, save for Billy who gets his head stuck in a bucket and is put out of commission. The electric fences are destroyed and all the Subterraneans break free, destroying the entire robot army. As everyone gathers to celebrate, one final UFO freezes everyone in a block of ice and the leader reveals himself: a robot version of Billy Dilley. Billy comes to and faces his evil doppleganger with the fate of Subterranea-Tania at stake.
After a rather comically pitiful fight, Billy notices that Evil Billy has a literal "screw-loose" on his head. Billy attempts to fix it with his electric screwdriver, but accidentally destroys Evil Billy's dome causing him to violently explode. Anaximander, who was not frozen, frees everyone and Evil Billy turns out to be a small worm. Everyone wants to destroy it, but Billy stops them as he sees the beauty in everything. The worm dies happily and brings the lushness of Subterranea-Tania back. Everyone forgives Billy and Judy suggests that he and his friends stay, but Billy decides it is best for them to leave. However, Billy, Zeke and Marsha are shocked to see the Cheeserator gone. Back on the surface of Earth, Anaximander uses the Cheeserator to get a taco at the drive-thru.
- Aaron Springer as Billy Dilley / Evil Billy
- Catherine Wayne as Marsha Dwanelle
- Tom Kenny as Zeke McBongo / Thurston
- Sarah-Nicole Robles as Judy
- James Arnold Taylor as Count Raymond Wretcher
- Susanne Blakeslee as Hag Witch
- Kenny Pittenger as Tony
- Richard Steven Horvitz as Yucky
- Travis Willingham as Gregory P. Zartran / Robot
- Kevin Michael Richardson as Gumbrump
- Fred Tatasciore as Ogre
- Roger Craig Smith as Robot
- This episode features the most actors in the cast than any other episode.
- This episode, and the series, ends on an unresolved cliffhanger.
- The robot army resemble the Starmen from the video game Earthbound.