On Halloween night, Dermott and Hank dare Dean to go into a nearby haunted house. Meanwhile, Orpheus, Alchemist and Jefferson host the Brimstone Assembly while Rusty and Hatred take bets on whether tricker-or-treaters will make it through the defense system.
In the aftermath of the Venture Halloween party, Rusty works desperately to complete a subcontract job for J.J. However, he recruits a team of interns which soon start displaying odd mutations. Meanwhile, Dean makes some startling life changes after Triana dumps him, and Twenty-One tries to convince the Monarch that he's no longer working for him.
When an old invention develops a mind of its own, Manhowers sends it to Central America; the Ventures must survive the jungle.
Twenty-One recruits some new henchmen to join his SPHINX organization, unaware that their members of the old villainous SPHINX. Meanwhile, the Monarch and Dr. Girlfriend infiltrate the Venture compound posing as beaver inspectors in a bid to plant explosives and destroy Rusty once and for all.
Dr. Venture takes the team along for his annual visit to the Greek island of Spanakos for the Spanakopita festival. However, Sgt. Hatred soon suspects that there's a dark secret behind the island festival. Meanwhile, St. Cloud travels to the island to defeat Billy once and for all.
The OSI manage to capture Monstroso and Molotov. But things go horribly wrong and afterward Internal Affairs tries to work out what really happened.
Hank and Dermott's prank on Rusty goes horribly wrong, and Hank gets himself committed with Twenty-One to fix things. Meanwhile, Dean goes to visit the secret woman in his life, and Dr. Orpheus makes a dramatic entrance.
When the OSI identifies one of the Council of 13 members, they send a civilian undercover to get close to him. However, the mission goes horribly wrong when their volunteer ends up at a villain club along with the Council of 13 and... Rusty Venture.
When Rusty is apparently killed by the Monarch, Hatred sends the boys off to live with their godparents, and then turns to Twenty-One for help to infiltrate the Flying Cocoon and recover Rusty's body.