Phil and Pim's constant bickering leads their parents to try a technique called “Versa Day”: the two Diffy kids switch bodies to see things from another perspective. While Pim decides to make the most out of the situation by embarrassing her older brother, Phil takes up for his little sis after an encounter with some mean girls at school. In other happenings, Lloyd tries to get Curtis a job.
Phil wonders if he has fallen in love when he becomes jealous of a cute waiter named Robby (Dossett March) who flirts with Keely at the mall; Pim is determined to join a clique cooler than the “fashion zombies” after not getting invited to Candida's slumber party.
Phil and Keely fear that the failing grades that they received on their history tests will detrimentally impact their lives; and Pim sneaks into an R-rated movie with help from an oblivious Curtis. Samantha: Anneliese van der Pol. Sally: Kyla Pratt. Susie: Fran De La Rose.
The Diffys invite their new next-door neighbor, Vice Principal Hackett (J.P. Manoux), to their home for an average 21st-century supper. Unfortunately, just as Mr. Hackett arrives, Phil's parents get sucked into a vortex caused by the Time Crumpler, leaving Phil and Pim with the unenviable task of entertaining their overly suspicious dinner guest.
Phil pushes for Tia (Brenda Song) to become Keely's new best friend, mostly because he is bored of going to the mall. Meanwhile, redecorating Pim's bedroom brings Barbara and Pim closer together.
Keely is stuck looking for another escort to the school dance after Phil is forced to break their date because his family is traveling back to the future; Pim transforms herself into a supermodel.
Lloyd's latest idea to get his family back to the year 2121 involves writing his future self a letter for Phil to put in a time capsule at school. The only downside to this plan---if it works---is that all memories of Phil's friendship with Keely will be erased. Meanwhile, Pim makes money raising funds for a not-so-sick rat. Ben Savage appears as himself. Vice Principal Hackett: J.P. Manoux.
Curtis accompanies Phil on his class trip to a museum, during which they come across a missing piece to the time machine amid a prehistoric exhibit; and the new substitute teacher, Ms. Mip, presents a huge challenge for Pim.
Phil and Keely, who have volunteered to babysit a little troublemaker, use one of the Diffy's gizmos from the future in hopes of improving the child's behavior, but (of course) things don't go as planned. Meanwhile, Pim's latest money-making scheme involves selling homework.
Keely pretends to be a math whiz, with Phil's help; Pim plans on teaching Candida the dangers of messing with static electricity.
Phil's refusal to complete a career-placement test lands him in detention under the supervision of Pim, who abuses her position of authority.
Pim resorts to dirty tactics to win the title of “Principal for a Day” against her opposition, Vice Principal Hackett. It comes as no great surprise that power goes straight to Pim's head, forcing Phil and Keely to come up with a plan to remove her from office.
Phil's friend Andy (Orlando Brown) arrives from the future and offers to take the Diffy family back home in his time machine, but only on one condition: Phil must help take part in a mean prank on the town mayor.
Phil gets his first real taste of independence when he's forced to move into the garage after Lloyd accidentally destroys Phil's bedroom.
Phil and his family become local celebrities when it's revealed the Diffys are visitors from the year 2121. While fame has some nifty perks, it cannot repair the rift that develops between Phil and Keely soon after he's thrust into the spotlight.
Keely helps the Diffys decorate their Christmas tree and learns of the first time she and Phil actually met in Pickford. It occurred during the placing of the star upon the tree.
Keely's matchmaking backfires when she tries to set Grady (Jason Earles) up with Grace (Ashley Drane), only to find that Grace's true crush is Phil.
Keely wants to improve ratings for her investigative news show, so she turns to Pim for advice on how to pique students' interest. Pim's tips work, but Phil worries that Keely is sacrificing too much to become popular.
Phil is determined to create an unforgettable birthday for Keely; Pim becomes smitten with a new student with whom she has a lot in common.
Each of the Diffys experiences changes in personality and appearance as a result of a futuristic disease called “greenemia,” which causes Phil to act like Pim, and vice versa.
Phil's Wizrd falls into the wrong hands when Danny finds it in the school courtyard; Pim helps her big brother Phil retrieve his futuristic gizmo.
Phil and Pim meet their great-great grandpa, who's a lover of not-so-great practical jokes.
Phil's relationship to Keely is abruptly altered by the revelation that the time machine is fixed, which means the Diffy family can finally go back to the future.