The Cheerleaders of East High School spiritedly welcome the students back from winter break ("Wildcat Cheer"). Gabriella Montez, a new student, tells Taylor McKessie about her ski trip; Gabriella flashes back to the New Year’s Eve karaoke contest where she met Troy Bolton and they sang a duet together ("Start of Something New"). Troy tells Chad Danforth that he made a New Year’s resolution to ask Gabriella out.

That morning the students – including Drama Club president Sharpay Evans and her brother Ryan Evans – gather in homeroom, where Ms. Darbus announces the auditions for Juliet and Romeo, the winter musical written by student Kelsi Nielsen. When Gabriella, Sharpay, Ryan, Troy, Chad, and Taylor use cellphones in class, Ms. Darbus sentences them to detention later that day. In the hallway after homeroom, Troy and Gabriella are shocked to discover they now attend the same school.

At basketball practice, Coach Bolton runs drills with the Jocks ("Get’cha Head in the Game"), but Troy’s mind strays to his duet with Gabriella. That afternoon, Gabriella demonstrates her intelligence in Ms. Tenny's chemistry class and Taylor tries to convince her to join the Science Decathlon team. Meanwhile, Sharpay and Ryan hatch a plan to trick Gabriella into joining the team to keep her from auditioning for the musical and threatening their stardom. Later, at Ms. Darbus’s detention, Coach Bolton bursts in, trying to find out why his players aren’t at practice, and finds the students doing acting exercises.

The theater is buzzing the next afternoon as Thesepians, including James, Susan, Cathy, and Cyndra audition for the musical ("Auditions"). Sharpay and Ryan audition for the lead roles with a creative take on a song Kelsi wrote for the musical ("What I’ve Been Looking For"). Troy and Gabriella arrive too late to audition, but Kelsi encourages them to sing her song as written, and the two of them perform as Ms. Darbus listens from the back of the auditorium ("What I’ve Been Looking For – Reprise"); she agrees to give them a callback.

The next morning, Sharpay is furious to see Troy and Gabriella on the callback list. In the cafeteria, members of the various Cliques are alternately inspired and dismayed at Troy’s transcendence of his clique ("Stick to the Status Quo"); Zeke Baylor reveals to the Jocks that he likes to bake, and Marth Cox tells the Brainiacs that hip-hop is her passion – much to Kratnoff's dismay.

Escaping the chaos, Gabriella and Troy share their fears and reminisce about how simple things used to be. Meanwhile, the Jocks and Brainiacs devise a plan to get Troy and Gabriella to focus on basketball and the Science Decathlon, and Sharpay hatches her own plan to take them out of the running for the musical: She will convince Ms. Darbus that Troy is trying to sabotage auditions as revenge for putting him in detention.

Later that afternoon, the Brainiacs try to convince Gabriella to concentrate on the Science Decathlon while the Jocks remind Troy to keep his head in the game ("Counting on You"). They put their plan into action by tricking Troy into saying that Gabriella and the musical don’t mean anything to him while secretly broadcasting this proclamation to Gabriella.

The next day, Troy apologizes to Gabriella, and they decide to participate in the callbacks after all. Their plan is complicated when Jack Scott, the school announcer, proclaims that the time of the callbacks has changed. Sharpay had succeeded in convincing Ms. Darbus to change the time of the callbacks to conflict with the Science Decathlon and championship game. Troy and Gabriella both say that they will support their teams, but the Jocks and Brainiacs feel guilty and devise a new plan to help them make it to callbacks ("We’re All in This Together").

The following day, all three events unfold at the same time ("Bop to the Top"): Sharpay and Ryan perform their callback while Troy plays in the championship game and Gabriella competes in the Science Decathlon, led by the Moderator. Taylor hacks into the school’s electrical system and shuts down the lights in the gym and lab to give Troy and Gabriella a chance to run to their callback. But when they arrive at the auditorium, they’re too late.

However, when the East High students arrive to rally behind them, Ms. Darbus relents. Sharpay protests, but Ryan reveals her lie to Ms. Darbus. Troy and Gabriella perform their callback ("Breaking Free") and get the parts. Troy and Gabriella return to their other competitions, and both teams win! The barriers between the cliques have fallen, and all of the students come together to celebrate ("We’re All in This Together – Reprise, High School Musical Megamix – Bows").