The following information may contain Spoilers. Please proceed at your own risk.
Jesse's Gang alongside Harper climb the stairs to find a large, void-like world which Harper calls "The Old Builder's World". As the group reaches the top of the stairs, Harper pushes them through a portal which leads them to the middle of an apparent game of "Spleef".
During this battle, Jesse's friends are trapped by members of the Green Team and Jesse must save either Petra or Ivor; whoever he/she does not save, along with Lukas, is killed. Enraged, Jesse knocks down Nell, who eliminated Lukas and Petra/Ivor, and throws back a block of TNT at a Gladiator, Slab (Character), whose fall halts the Game.
Jesse's Gang are then introduced to the Old Builders: Hadrian, Mevia and Otto who explain Jesse's friends have been sent to a re-spawn zone to work in the Quartz mines, and that the event is known as the "Games" where participants must stay until they win. Jesse demands the Atlas but Hadrian refuses. However, Jesse strikes a deal that if she wins the game, her Gang can leave with the Atlas.
In the Competitor's Village, Jesse is able to convince a Gladiator, Facemeat to hand over Lukas and Petra/Ivor with the help of Nell, who tells them about "Tim"- a legendary hero who won the games and took his friends home.
The next day, the Games feature "[]", a survival-run minigame where players must go through obstacles. The Green Team's leader, Emily offers Jesse a chance to team with them, which Jesse can accept or decline. However, either way Em pushes Nell of the vines and (determinately) betrays Jesse. Jesse can either save Nell or win the games.
However, in both outcomes, Hadrian will eventually not go by his word and leave the group to face Em, who walks away. Slab then comes to tell Jesse that Hadrian was calling her and he needed to report to the palace. In this situation, Jesse can either do as Slab says or can insult him and delay their trip to talk to Sebastian.
In the palace, Hadrian reveals that he has Axel, Olivia and the EnderCon Usher, Reuben hostage and that if Jesse does not lose the games, they will remain with him forever. Determined to win, Jesse, back in the Competitor's Village, gives a speech to the crowd, inspiring them to come together against the Old Builders. Em also agrees to be a part of this.
The following day, Jesse is alone while participating in another minigame, "The Walls". Her section, however, is rigged and does not have equal resources. Ultimately, Em helps Jesse and Jesse convinces all the teams as well as the Gladiators to come together against the Old Builders.
In a final standoff against Hadrian and Mevia, Jesse dies but it is revealed Em has freed her friends who give her Tim's armor, weapons and tools. Jesse then is able to take down the duo with her friends' assistance as Otto gives her the Atlas.
Finally, the group reaches the Portal Hallway where Slab determinately returns Lukas' journal which he dropped when he died depending on whether Jesse was nice to him. Ivor says goodbye to Harper as Otto promises Jesse he will restore the Games to their former glory. Em also determinately accompanies the group back to the Overworld. Back home, Jesse and the Gang admire the treasures they collected during their adventures. Inspired, Ivor runs back to the Portal Hallway as the credits cut in.
In a post-credit scene, Hadrian and Mevia are seen in a world full of either chicken-sized zombies or zombie-sized chickens (as per the choice made in the beginning of the series).