After Grim Jr. is fused with the Pumpkinator and his inner Nergaling has taken control over his body he makes his way to Halloween Town to create chaos. Jack Skellington then commands Sally to get everyone save while he tries to defend the town.
The Demon Reaper enters the town and starts attacking Grim, Mandy and Jack with it's tentacles, but the three manage to dodge and cut them. Then Lord Pain arrives on the scene with the Royal Guards who start to attack the monster. Lord Pain is ordered to summon the Beast of Destruction, but unfortunately it gets eaten by the monster. Mandy then orders to summon the Archer but they are interrupted by Dan Phantom who fires a giant ghost blast at the monster, which blows of it's husk. While Dan continues to fight the monster, Clockwork appears before Grim and tells him they have to eliminate the core of the monster. Dan does a ghostly wail which allows Clockwork and Grim to enter the monster. While inside they encounter Nergal with whom Grim has a fight with about Minimandy. Nergal tells Grim that he isn't her real daughter, else she would be a skeleton, he also comments on every abortion being a child of Grim, as Grim Jr. is too. The core of the monster appears and Grim reaps out the soul of Grim Jr. and Nergal leads them to the exit. Clockwork takes over Grim Jr. while Grim holds off the tentacles. And the two eventually make their escape.
Grim arrives back in Halloween Town but sees that Clockwork has taken Grim Jr. along with him to somewhere else. Mandy gets mad at Grim but comments that they have to continue fighting. Grim Jr. on the other hand was brought by Clockwork to Hell so he could find his sister and learn more about himself. After Clockwork disappears he encounters Redeemer.