Penn wakes up from a nightmare to find his aunt and uncle filming him, they explain they have been doing it to show his parents. He then finds sashi, Boone, Phyllis, and a old man with a walker in his room as they prepare to save Penn's parents from the Most Dangerous World Imaginable.
Phyllis introduces Penn to the first part-time hero, the old man Adam Tomkat Badawy who tells him a story to try to minimize his self doubt and give him confidence.
When Penn misses the point of the story Adam tells him he should believe in himself and listening to his "inner hero", before flying through a wall with his rocket-powered hi-walker.
At the theater Penn, Boone, and Sashi get hi-tech battle suits, with Boone's having a helmet that gives him the advice from the wisest people in history, Sashi's having lots of weapons, and Penn's having a housing for the shards. They all have rocket boots.
Phyllis then explains in order to locate the Most Dangerous World Imaginable they must use the three shards they collected to gather evil energy from the most dangerous spots in the three most evil worlds, and when all the shards are charged, Penn's suit's triangulateor will open up a portal which he will be able to pull his parents out from. They were unknowingly being spied on by Rippen, Larry, and Phill via-a-robotic mouse planted in the theater.
The first world is a large tunnel with strong winds, when they get close to the most dangerous spot they are attacked by a windmill-like monster who reverses the wind breeze to shed the heroes.
When Boone's helmet doesn't give him good advice, Penn encourages him to use his own wisdom.
Boone then uses his helmet to jam the monster's blade causing the top of it's head to explode, and after gather enough evil energy in a shard go to the next world.
In the next world rain rises from the ground and are confronted by a sentient reverse-flowing waterfall.
After accepting it's challenge, Sashi is force to fight three evil clones of herself while Penn and Boone deal with their own clones.
Sashi manages to beat her clones revealing she made her left shin a weak spot in case she had to fight herself one day.
With the second shard charged, they go to the last world where a bunch of pieces of land float over a sea of fire. Before they can get to the most dangerous spot atop of a mountain's summit; Rippen and Larry block their path and zap in a army made up of villains from the worlds they visited.
Luckily Phyllis appears revealing she planted a camera in Phil's best customer, Old Man Middleburg's beard, and bringing their own army made up the friends they made throughout their missions.
Phyllis and Phil then explain that if the villains win the balance of power in the multiverse will tip towards evil meaning that evil will always win.
During the battle, Penn is able to get to the mountaintop but is struck from behind by Rippen causing the shards to lose their energy.
When Rippen tries to finish Penn off, his evil inadvertently charges the shard and a portal to the Most Dangerous World Imaginable opens up. However instead of pulling his parents through the portal, Penn decides to listen to his "inner hero" and saves Rippen from falling off the side of a cliff, only for the shark monster from ocean world knocks Penn off cliff as Rippen earlier ordered it to knock anyone besides him off the mountain.
As his life flashes before his eyes, Penn accepts his fate as he had a good life.
Fortunately Boone and Sashi manages to save him from falling into a fire monster's mouth, but when Brock and Vonnie who were surprisingly pulled through the portal by Rippen, jump off after Penn, are missed by Boone, they land on Sashi, who's suit boosters couldn't support the weight, causes them to fall again.
They were then saved by Adam. When Boone falls on them Adam's rocket-powered hi walker gives out and they fall yet again only to be save by Phyllis and a currently giant bear Karen.
With the heroes having won, Phyllis then explains that they need to go to the Most Dangerous World Imaginable and close the portal using implosion bombs stored in side Denis the robot's body, or else all worlds will be destroyed.
After sending most of the heroes and villains back to their own dimensions, those remaining go the Most Dangerous World Imaginable to plant the bombs while fighting off it's monsters, during which Scaley Briggs reveals he going to retire and make Amber the new sheriff of their dimension.
All the bombs are planted but when a giant chicken starts pecking at the main bomb, Rippen takes over the body of a giant ogre by going into it's head while also revealing he has a tail to beat it before knocking the ogre out with it's own fist and scaring the other monsters away with a simple "boo".
Everyone zaps back to the Odyssey, but when the bombs fail to be remote detonated; Rippen reveals earlier in the day he snuck in and sabotaged a bunch of stuff.
With the multiverse being ripped apart, Phyllis says the only way to close the portal now is for someone to go there and closing it from the other side resulting with the person being trapped there.
Brock and Vonnie volunteer to go but are stop by Rippen who decided to go there himself after learning the value of integrity from Penn and his friends. Rippen sets out to conquer the Most Dangerous World Imaginable.
Lady Starblaster, having been impressed by Rippen, decides to go with him, along with Larry who want to be Rippen's minion forever, -and after calling his pet tea cup elephant Tony and giving Old Man Middleburg the deed to his ancestral mansion- the president from Giant Monster World also decides to come along to be with Larry.
With Professor Evil Professor, Dr. Hissy, Mayor Direginald Hindenburg, Sonny and Ms. Monkenfluffer who just show up deciding to go too, they go through the portal and Rippen and Larry detonate the bombs closing the portal and saving the multiverse.
Afterwards, they sent Maria, Captain Super Captain, Scaley, and Amber back to their respective dimensions and accidentally sending Adam to water world.
Later when Brock asks Phil to get some new part-time villains to humiliate daily, Phyllis says there is no need as she give everyone but Boone a resume for their next job, stating with balance restored there's no longer a need for part-time heroes or villains.
Penn realizing their job is done asks Phyllis what they should do now with Phyllis stating to do what they always done; be heroes. Phyllis and Phil then say that the most important thing is balance while fusing into a cosmic being know as the Guardian.
The Guardian tells the group that it will always be watching, probably, especially if a hero is needed, before vanishing with the Odyssey and Fish Sticks on a Stick floating into the air and imploding.
With everyone confused whether they were the same person or became the same person, Vonnie states maybe the forces of good and evil "just needed a good shake". When she asked if that made sense, Brock says it didn't. Vonnie notices their car, which is covered with parking tickets, which she'd thought would have been towed by now. Penn say the important thing is that they can be a family again.
Brock is sleeping on the couch with Vonnie sitting next to him, Denis the robot is now living with the Zeros and Penn is happy to finally hav his parents back.
Boone and his parents are having a meatloaf dinner and Boone gives a large piece of meatloaf to Karen the bear who now lives with them.
Sashi's specs are put on the family trophy shelf with her parents both hugging her.
The scene then goes to different dimensions.
First shows in Ultrahyperball World that Coach Jackie and Coach Wallace now have eight kids: four blue and four red.
In Knight World, Ryan and Little Boone are riding Nathaniel and some goblins riding a dragon play jump rope with a bird monster. In Anime World, Arata, Hideo, and Yumi are riding the ape-bus. In Space World it show two aliens flying a space ship.
In the Most Dangerous World Imaginable, Rippen and everyone who decided to go with him, along with Piv and Gree, arewaiting for what's left of the giant fire-breathing chicken to get done roasting over a volcano. While they're waiting Rippen decides to go over their evil agenda and Larry says no matter how many evil battles they go through at the end of the day they have dinner as a evil family. Lady Starblaster is sick of Larry's optimism and tries to blast him.
The President blocks the blast and everyone tries to take cover from the laser fire.
Though hectic Rippen says that it's still better than teaching.
In Rock, Paper, Scissor World, Queen Igneous, the Scissorian General, and a Papyron all having a relaxing time together.
In Pirate World, Maria has made up with her old crew and are playing a game of cards onboard boat Maria.
In Alien World, Princess Alrgelbaccch Blunkenthorttthhhph and Grinkon are out on a date.
In Ball World, Bowling Ball and Eight Ball get married.
In Middleburg Rose, Chuck, and the Chinchilla now work for Old Man Middleburg at Larry Manor.
At the high school Penn, Boone, and Sashi gone on to have normal lives, with Sashi letting her hair down.
In Musical Fairytale World, the Princess and Queen are living happily together.
In Superhero World, all the heroes are enjoying tacos and when Captain Super Captain tries to give Super Dog a taco, Super Dog ends up biting his arm.
In Dinosaur Cowboy World, Scaley enjoys his retirement, while Amber is enjoying her new role as sheriff of Big Butte.
Later, Penn, Boone, and Sashi are walking together. Sashi says that it feels weird not going to the theater while Boone says that they should just move on and he decides to grow a beard when he can. Remembering how Phyllis told them to keep being heroes Penn decides to make their own missions. Seeing a flyer for a lost puppy, Sashi thinks that finding it could be their mission. Boone goes out to find it, wanting to name it Boone Jr. or Michell. When Penn tries to go after him to explain that it already has a name, Sashi stops him. After a brief moment of silence, she kisses him and he kisses back. When Penn blushingly asks if this is a thing now, Sashi says "Maybe" before hitting him on the arm. As Boone comes back having found the puppy, he and Sashi walk off while Penn says that they're going to need to find some tougher missions before following them.
The scene changes to the Guardian who was watching them, talking to someone in an alien language saying "Phase One" is complete, before asking to commence "Phase Two". A heavenly choir is heard as the screen show the Guardian's smiling face as the series ends.
- Penn Zero
- Brock Zero
- Vonnie Zero
- Sashi Kobayashi
- Boone Wiseman
- Adam Tomkat Badawy
- The Guardian
- Old Man Middleburg
- Karen the Bear
- Sheriff Scaley Briggs
- Amber Briggs
- Giant Old Lady
- Mr. Egsgard
- Super Amazing Bouncy Ball
- Captain Super Captain
- Island City President
- Boat Maria
- Dr. Hissy
- Mayor Direginald Hindenburg
- Sonny Sonnerstein
- The Butterman
- Windmill Monster
- Evil Waterfall
- Princess Alrgelbaccch Blunkenthorttthhhph
- Shark Monster
- Lady Starblaster
- The Snowman
- Rogue gingerbread men army
- Mayor Pink Panda
- Professor Evil Professor
- General Bighorn
- Evil octopuses
- Three-eye cat chef
- This episode was posthumously released two months after the death of actor Adam West, who provided the voices for Captain Super Captain and Professor Evil Professor.
- Penn & Sashi officially shared their first kiss.
- It could be possible that writers had intended for Penn and Sashi to end up together.
- Worlds seen/Visited: Most Dangerous World Imaginable.
- It's revealed that Phyllis and Phil are the same creature called The Guardian.
- Lady Starblaster's first name is revealed to be Ami.
- Brock and Vonnie are finally back in Middleburg.
- Rippen fail missions for about 1,665 times.
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