Power Play is the fourth book in the Sweet Valley High book series. 

Robin Wilson, a newcomer to Sweet Valley, is an obese girl who is intent on joining Pi Beta Alpha, the most popular and prestigious sorority at SVH.  She also thinks that Jessica Wakefield, the sorority's president, is her best friend. 

Jessica has always made certain that PBA would only take in the most popular people, and to exclude any undesirable people....people like Robin!

When her twin sister, Liz nominates Robin for membership, Jessica is truly enraged.  She doesn't see Robin as Pi Beta material, and is, as usual, only concerned for appearances.  She sees PBA as a group that many aspire to, but only a select few can attain.  However, she takes the nomination, and she and her two friends, Lila and Cara personally plan to take Robin through her pledging period. 

However, their scheme is to haze Robin so mercilessly, that she would want to drop out of the pledging.  Among other things, they make her jog around the track, to the derisive amusement of the rest of the school; make her wear a bikini to the beach; and also to have Bruce Patman bring her to the Discomarathon dance.  (Bruce having brought her due to some uncharacteristic blackmail done by Elizabeth)

Bruce does bring her, but then humiliates her in public by calling her the Queen Mary!  Elizabeth is enraged at Jessica, Lila and Cara for their snobbishness and tries to help Robin during this time.  The vote came up, and Jessica got even with Elizabeth for daring to nominate Robin, by blackballing her.  Robin was told the news by Jessica, who did not hide her glee.  This proved to be too much for Robin, and she shut herself from everyone in Sweet Valley High, all the while, shedding the weight.

In the book's B-Storyline, Jessica is receiving lavish gifts from Lila, which makes Elizabeth suspicious.  While at the mall, she sees Lila shoplifting from Lisette's, an expensive boutique.  Elizabeth confronts Lila about it, and she discovers the truth about Lila.  Lila reveals to Jessica's twin, that she is incredibly lonely. With her father constantly away on business all the time, she feels that she doesn't have his attention.  Her only confidante at home is the Fowler's housekeeper, Eva.  She was stealing from Lisette's to get his attention. 

George Fowler, Lila's father, makes restitution, and Lila is put on probation.  The Fowlers and Elizabeth go and eat at the Palomar House, one of the top restaurants in town.  Jessica is trying to find out what is going on with Lila, but Elizabeth, true to her word to Lila, wouldn't reveal the story to her.

After the shoplifting episode is taken care of, the Miss Sweet Valley High contest was in full swing; Jessica was certain that she would win, and was planning to be the winner.  Then, Robin, who had shed the weight, and was now the cheerleading co-captain, along with her nemesis, Jessica, made an accusation that the Pi Betas would not allow her to be in the running because she had been blackballed by them.  The accusation threw the Pi Betas off guard, not expecting that someone would do that to them.

Then, at the homecoming game, it was announced that Robin had won Miss Sweet Valley High, to Jessica's complete shock.  Robin chose as her escort, Allen Walters, a photographer for the Oracle.  After all that, Jessica offered Robin membership in Pi Beta, but she refused, realizing who she really was, and that she didn't need the sorority to define who she was.