Carly's show's 50th airing is coming up but Sam gets detention. Carly and Freddie do bad things to get in detention to shoot the show from there. Meanwhile, Spencer drops his phone into a large coffee cup built by him

Carly's old friend Missy Robinson returns to Seattle and tries to become Carly's only best friend and replace Sam because she was Carly's best friend first. Sam was given expired chocolate by Missy, and she destroyed Sam's cell phone, and in addition, Missy texted Sam the wrong address to a helicopter base, that she intended to go on with Missy, and Carly. Missy's actions are claimed "accidental", however Sam realizes what Missy is trying to do, but no one believes her as they think Sam is being "her usual paranoid self", because she was being paranoid when she thought Lewbert was an undercover cop, because she thought his giant wart was a spy cam and thought Gibby was a mermaid because he doesn't like wearing shirts. Missy confesses to Sam, but since no one will believe her, she is forced to ask Freddie for help, so when Freddie wins a six month cruise, he gives it to Missy so she will go away. Shortly after Missy is awarded with the cruise, Carly discovers Missy's plans to destroy her and Sam's friendship. Meanwhile, while packing for a camping trip, Spencer is locked in the basement by a psychotic boy, Chuck, seeking revenge from Spencer for telling on him and getting him grounded in the episode "iHurt Lewbert". Chuck is eventually caught at the end of the episode and is grounded by his father.

Sam and Freddie are forced to share a big locker after they both answered how many Fat Cakes that were in the inside the container. Meanwhile, Carly decides to take drawing lessons from Spencer, but she becomes frustrated with her brother's teachings. Carly decides to take an art class in secret, but Spencer finds out, he embarrasses Carly and he tells the art teacher that art students should be spontaneous and passionate. The art teacher finally takes Spencer's words into consideration and they share a passionate kiss. Sam decides to let Freddie have locker 239, she takes his money and she goes home with her mother after getting laser eye surgery. As a result, locker 239 gets destroyed, courtesy of Mrs. Puckett's reckless driving, Freddie gets in trouble for the incident.

Sam has a twin sister named Melanie (Portrayed by Jennette McCurdy). Freddie is not convinced, and to try to prove it, he sees Melanie in person and invites her to a dance with him as his date. The plan to try to prove that Melanie is really Sam takes an unexpected turn when Melanie kisses Freddie, who then becomes extremely paranoid. Meanwhile, In the end, since Freddie refuses to believe it, Sam admits to Freddie that there is no Melanie. Satisfied, Freddie departs right before Melanie (who reveals that she still has romantic feelings for him) arrives to go to the mall with Sam and Carly. Meanwhile, Carly tries to tutor Chuck in math, and Spencer shows Carly how evil Chuck really is (with the help of a hidden video camera that Socko set up). Carly gets revenge on Chuck by telling him there's a new number called "dirf" between 5 and 6, and because of this, Chuck fails his math test and Spencer gives his dad a brochure about a summer math camp called Camp Add-m-Up, sending Chuck to math camp for the whole summer.

Principal Franklin is fired after appearing as a guest on iCarly; Spencer hires a rodeo star to teach him how to ride a mechanical bull.

Carly is dating a boy named Steven who divides his time between his divorced parents who live in Seattle and Los Angeles. Meanwhile, Tori is also spending time with her new boyfriend who, unbeknownst to her, is the very same boy Carly is seeing!

Sam and Gibby secretly operate a restaurant in the basement at school, while Spencer creates a security robot that goes haywire.