As the students are working on their science projects, Mr. Belding walks in with a new girl, Jennifer. Slater tries to avoid her and it turns out that she's his old girlfriend, Jen, from Germany. They used to live next to one another on the base where their parents were stationed. He never broke up with her before moving and now has to keep this from Jessie.
Jen sees him and calls him by his full name, Albert Clifford. He informs her that it's A.C. now. Slater tries to keep the two girls apart so neither will find out about the other, but Screech breaks the news to Jen and Jessie finds out when she overhears Slater talking to Jennifer through her science project, a micro transmitter hidden in a pen.
Zack doesn't want Slater and Jessie to break up since he wants to date Jen, so he comes up with a plan to date Jennifer. This way Jessie will see that Slater isn't attracted to Jen anymore. Jessie also asks Zack to date Jen to test Slater's feelings for Jen. Screech ends up telling Jen that Zack is going to ask her out on a date so that everything will be OK between Jessie and Slater. Jen goes along with the plan.
Zack and Jen then show up at the Max and sit off by themselves. They act romantically toward one another, then go dance on the dance floor. Jessie goes to the bathroom and tells Zack he's doing great. Then Slater cuts in, asking Jen if he can come by later to talk. She tells him to call as she is going to the beach with Zack. Slater goes over to Zack and yells at him for really dating Jen.
The next day, Kelly and Screech show their science project, a Love Machine, that tests the compatibility of two people. Jessie volunteers Zack and Jen. They register as Very Compatible. Slater gets upset and accuses Zack of stealing his girlfriend. Jessie asks him for clarification. Slater says that it's all the machine's fault and grabs one of the controllers, which sets off the machine, registering WOW!, the highest rating, Jessie runs out of the room, upset. Kelly follows.
In the locker room, Kelly tells Jessie to have Jen and Slater go out on a date, to see if they still have feelings for one another. Jessie agrees and tells Slater this. He ends up taking Jen to a Mel Gibson movie, which used to be one of her favorite actors. She tells him that she doesn't care for Mel Gibson any more and that she prefers to be called Jennifer. Slater tells her that he prefers to be called A.C. instead of Albie. They agree that they aren't attracted to each other any more.
Meanwhile, an older couple enters the theater, who are really Jessie and Zack in disguise, Jessie as the old man and Zack dressed as an old woman. They spy on Jennifer and Slater, disturbing the two, who move further down the row of seats. Slater gets a cramp in his neck from the headlock that Jessie had him in earlier that day. Jennifer offers to rub it, causing it to look as if the two are about to kiss. This upsets Jessie, who runs out of the theater, followed by Zack.
They go sit in a booth at the Max, depressed. Jennifer and Slater come in and ask Kelly if she's seen Jessie and Zack. The two reveal themselves. Slater tells Jessie that she's his girl and they leave. Jennifer and Zack make a date to go to the movies.