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 fiance 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 tells Timmy about how he must hide him from his fiance, begging Timmy to let him stay on Earth with him. Timmy allows this, but tells Mark to get a disguise. Pondering a bit, Mark Chang remembers that he can use his 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 is happily eating the contents of the garbage can, remarking how it's a good thing they serve it by the buckets there. Timmy then tells Mark that if he wants to blend in, he has to eat people food. As Mark follows Timmy, 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 Cosmo and Wanda about what to do with the alien, Mark then sees a slice of chocolate cake, concluding that it must be gross since it's brown. He eats it in one go, but upon eating it, he realizes that it is chocolate, a hazardous food to Yugopotamians. He spits it out, but it is too late--it 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 then allows Mark Chang to stay in the Dimmsdale Dump, where he reveals that if he were to marry Mandie, the Yugopotamians and the Bodacians would form a fearsome alliance and take over the Earth. Feeling grateful for Timmy's help, Mark decides to go against his nature to hate human emotions and instead, hug Timmy for his good deed. 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 was futile as Mandie was still hot on their trail. The only way to get rid of her was to get her to marry someone else.
Getting an idea, Timmy places 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 their problem was solved for now.
The next day at school, it is seen that all the girls are all 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 only because Mark was eating out of his garbage can.
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.