In anticipation for his birthday party, Cartman hands out invitations during class.
After he hands out the invitations, Mr. Garrison announces that a new student is joining the class.Damien Thorn introduces himself as from "the seventh level of Hell". He continues that, "Now is the beginning of the end and the new reign of Satan, the Prince of Darkness". After Cartman taunts him with a birthday invitation, Damien's eyes flame and he telekinetically throws Cartman's desk out the window, prompting Mr. Garrison to ask if he needs a "time out".
At lunch, it's revealed that the birthday invitations are marked with a gift that each student is to buy for Cartman:
- Stan: Green Mega Man
- Kyle: Red Mega Man
- Kenny: Yellow Mega Man
The boys continue to mock Damien throughout lunch, and he turns Kenny into a Duck Billed Platypus before going into a rage and destroying the lunch room, demanding Jesus be brought to him.
While the rest of the boys go and find Jesus, Cartman revises his birthday gift list, making it clear that he needs all the Mega Men in order to combine them and create Ultra Mega Mega Man.
- Wendy: Yellow Mega Man (formerly Mega Man Power Chopper)
- Clyde: The Mega Cycle
- (Unnamed children): Mega Man Beach House ("Two people at once since it costs more money")
Meanwhile, Damien's wrath continues on the playground during recess. When Jesus arrives, he tells Damien to let Satan come and he will face him. While waiting for Satan, the whole town manages to gather at the playground. Although Satan does not physically show, he informs everyone (through Damien) that the arrangements for the final battle will be made the next day, prompting Jimbo to make his way to his bookie, followed by the rest of the town.
It is decided that the final battle will be held at the South Park Forum and can be watched on pay-per-view for $49.95. Naturally, Cartman is angered that the final fight between good and evil will be held on his birthday. In anticipation of the fight; the entire town bets that Jesus will win, until Satan appears for the weigh in, appearing much larger than Jesus, sending the town back to the bookie to change their bets.
The boys take it upon themselves to train Jesus, drafting Chef into sparring with him. Chef reluctantly knocks out Jesus with a single punch.
At Cartman's party, Damien and Pip show up. In order for Damien to "warm up" to the boys, he opens a pit of fire and lets black ghosts throw Pip in the air where he is engulfed in flames. Impressed by the fireworks display (and for beating up Pip), the boys welcome him to join the party. While opening his presents, Kyle, who was supposed to give Cartman a red Mega Man, gives him the game Ants in the Pants, saying the stores were all out of red Mega Men. Enraged that he can't get the Ultra Mega Mega Man, Cartman attacks Kyle and promptly cancels the party, ordering everyone out; afterwhich they all go to see the fight.
At the fight, the referee informs Satan and Jesus that he, "Want[s] a good clean fight, guys, no punching below the belt, holding, or miracles." Satan lands punch after punch on Jesus. Jesus is disheartened by the town's lack of faith in him and does not attempt to retaliate as Satan wears him down. When Stan shows up from Cartman's party, he gives Jesus a small pep talk about how Nancy Kerrigan wouldn't give up until she was #1. Kyle interrupts him to tell him that Nancy got the silver medal (second place). Instead, Stan tells him, "Don't try to be a great man, just be a man." When Jesus asks who said that, Stan tells Jesus that he did (but Stan later admits he heard it on "Star Trek").
Inspired by Stan's words, Jesus punches Satan in the lower-forearm and Satan takes a dive, throwing the match. It is revealed that the only person who bet on Jesus to win was Satan. He had planned to inspire the crowd to bet on himself and get rich off their folly (and buy some real estate). The town asks for Jesus' forgiveness and he replies, "Aww heck, do I have a choice?" Damien says goodbye to Stan and Kyle who agree that happiness is what counts for children. Ironically, a sickly Cartman at the end is seen fatter than usual while his mom asks him if he wants more pie.