New Squid In Town! is the twenty-first episode of Season 4.
Mark uses his "image fake-i-fier" to look like a human and hides in Dimmsdale from Mandie, a homicidal alien princess girl whom his parents want him to marry. Mark soon finds adjusting to Earth proves difficult, as certain actions shortcircuit the device and reveal his true form. Worst of all, Mandie follows him to Earth, willing to make him marry her at any cost or kill him if she can't marry him.
At the peaceful Yugopotamian Church, the Yugopotamian prince Mark Chang and the Bodacian Princess Mandie's arranged marriage is being held. Queen Jipjorrulac is not only happy, but also scared, so she shrieks in both delight and sheer terror, and her husband seems happy too--he just wishes that Mark's fiancé wasn't so "hideous" (in Yugopotamia, everything is backwards, so Princess Mandie is actually gorgeous.) On the other hand, Mark is sweating nervously and is obviously not looking forward to his wedding. Ultimately during the marriage, Mark flees from the altar and states that he cannot wed Mandie for he does not love her. He then proceeds to fly off in his spaceship to Earth. Princess Mandie sheds a tear and Mark's parents try to comfort her by saying that he's probably just getting some sodas. However, Princess Mandie soon shows her violent side when she reveals that her name is actually pronounced Man-DIE, and she brandishes her flaming sword and threatens Mark's father to tell her the location of Mark.
Meanwhile, in Crocker's classroom, Timmy braces himself for another boring day of school. All of a sudden, Mark Chang's spaceship smashes through the roof of the classroom. Mark exits his spaceship and frantically calls upon Timmy to help him. They then remember about the presence of the rest of the class, who freak out upon seeing the alien. Mr. Crocker points at Mark and shouts how there is an alien in the class. Thinking quickly, Timmy wishes that every other human in the room would freeze.
With all the distractions out of the picture, Mark explains to Timmy and his godparents that he's hiding from "some girl [his] parents want [him] to marry." Mark begs Timmy, Cosmo and Wanda to help him hide out on Earth from fiancé, and they agree to help him, although Timmy points that Mark's probably going to have to disguise himself if he doesn't want people finding out that he's an alien and asks if Mark has any kind of alien gadget on him that would be able to disguise him as a human. Mark then remembers that he has a Yugopotamian-gadget called a "Fake-i-fier" to transform into a normal boy. Wanda reminds Timmy that he needs to unfreeze everybody, but before doing so, Timmy moves Crocker's frozen body into the girls' bathroom and decides to unfreeze him right when Principal Waxelplax walked in. As a result, Waxelplax becomes furious, rips the hand-dryer machine from the wall, and proceeds to beat up Crocker with it.
In the cafeteria, Mark Chang's happily eating the contents of the garbage can, remarking how it's a good thing they serve it by the buckets on Earth. Timmy advises Mark that if he wants to blend in better, he's going to learn to eat what Earthlings like himself and his schoolmates eat--this includes eating the food that's served in the school cafeteria. As Mark follows Timmy, Timmy's friend, Chester, is then seen happily eating the contents of the garbage can, also commenting about how they served it by the buckets there. After Timmy discusses with his godparents about what to do about Mark, Mark sees a piece of cake that's covered in brown-colored frosting, and concludes that it must be gross since it's brown before eating it in one bite. Unfortunately for Mark, the frosting turns out to be a chocolate frosting, and the frosting shorts out his image Fake-i-fier and his true alien form is revealed, causing kids to scream.
All of a sudden, another spaceship smashes through the ceiling and Princess Mandie emerges from the diamond-shaped ship. Walking up to Mark and wielding a fiery sword, she threatens him to marry her or die. Crocker then comes again and shouts again how there is an actual alien in the room. Yet again, Timmy wishes that everyone would freeze, including the two aliens in the room. Feeling sympathetic for Mark Chang, Timmy decides to poof him someplace else before unfreezing everybody. However, Timmy messes with Crocker's frozen body again by poofing him into Principal Waxelplax's bedroom while she is sleeping there. She becomes furious again, rips a random hand-dryer machine from the bedroom wall, and beats Crocker up with it, much like before.
Timmy and his godparents take Mark to the city's local garbage dump, which Mark loves. Cosmo and Wanda ask Mark why Mandie's hunting him down and why they were supposed to get married in the first place. Mark explains that if he were to marry Mandie, their home planets would form an alliance (the most fearsome in the galaxy/universe) and team up to destroy the Earth--however, Mark can't bring himself to marry Mandie because he doesn't truly love her and he also finds her unattractive. Feeling grateful for Timmy's help, Mark (going against the Yugopotamian-nature of hating things that Earthlings love) decides to give Timmy a hug as a way to thank him. During their embrace, however, Mark's Fake-i-fier shorts out due to the exposure to human kindness. Luckily, they were the only ones there.
Unfortunately, Princess Mandie quickly arrives at the scene and blasts them. Using her hi-tech goggles to detect the energy levels of the four, she discovers that Cosmo and Wanda are the most powerful and decides to take them out first. Using her flaming sword, she freezes Cosmo and Wanda in a solid block of carbonite. Scared that they will be next, Mark and Timmy flee to Mark's Yugopotamian spaceship. Seeing that they were fleeing from her, Mandie screams with rage and frustration and she chases after them. Timmy begins firing bullets from the cannon of the Yugopotamian ship, only for Mandie to dodge them, causing him to destroy the ice cream shop, the video game store, and the comic books shop, all the stores nearby decimated except for, much to his horror, the "Girly Things" shop. Mark and Timmy realize that running away from Mandie is ultimately futile, and after Mark points out that they only way to get Mandie off their trail is to somehow get her to marry someone else, Timmy then gets an idea.
Timmy and Mark place another Fake-i-fier outside of Crocker's house, pretending it is a fairy-hunting belt. Filled with excitement, Crocker puts on the belt and abruptly transforms into a Mark Chang lookalike. Descending from the vast night sky, Princess Mandie nabs Crocker and flies off with him. Mark and Timmy then emerge from behind Crocker's house, happy that both of their problems are solved (for now at least). Seeing how they averted danger, but with Mandie still out there, Mark ultimately has to hide out on Earth for a while--but Timmy points out that Mark's going to have to try and be less inconspicuous and cause a lot less trouble if he doesn't to be found out, with the alien prince promising to do so.
The next day at school, it is seen that all the girls are fawning over Mark, who was telling them not to kiss him since their earthly affection would short out his transforming device. His bluntness make the girls swoon and want him even more, and he gets a lot of attention. The only one a little annoyed by Mark's sudden appearance is Chester, but decides to give him a chance as long as Mark doesn't eat out of his garbage can again.
Meanwhile in Yugopotamia, Crocker in his Mark Chang-form and Princess Mandie are getting married. Mandie gives Crocker his wedding kiss, shorting out the Fake-i-fier and revealing Crocker's true form, shocking all the Yugopotamians. Mandie yet again begins to growl and get angry, so she throws him right into Principal Waxelplax's house, waking her up. Upon seeing Crocker, she once again rips the hand-dryer machine off her wall again and uses it to beat up Crocker with zest and zeal.