Tensions are high between sisters Tegan and Sara on their first day at a new school. Their relationship became fractured over the summer because Tegan felt cast aside by Sara and their shared best friend Phoebe. As Sara tries to break free from Tegan, Tegan tries to make sense of Sara pulling away and attempts to make friends outside of her sister for the first time.

While Sara continues to withdraw from her family, especially her mom, she and Tegan make headway with friends at their new school and are invited to a party. But as Tegan begins to make friends of her own, Sara starts to realize how important Tegan is to her.

As Tegan becomes closer to Maya, she gains more perspective on her relationship with Sara. Tegan feels responsible for Sara at all times and it starts to weigh on her, especially compared to her friendship with Maya, which feels easy. Meanwhile, Sara struggles to balance her new friendships and interests with her relationship with Phoebe.

While stuck at home, grounded, Tegan and Sara discover Patrick's old guitar, and learning to play it becomes their new obsession. Not only does this launch them on their journey as musicians, but their newfound shared love of playing music begins to repair their fractured relationship.

As Tegan and Sara begin writing songs together, they start to bond and grow closer in a way they haven't been all year. This complicates Tegan's relationship with Maya, as Tegan chooses to spend more time with Sara than with her.

As Tegan tries to repair her friendship with Maya, Sara makes an effort to ensure that Tegan feels included by her and Phoebe by introducing Tegan to Phoebe's friend Cass. Tegan and Sara start performing their songs in front of more and more people, leading to an exciting opportunity and chance for them to take their journey as musicians to the next level.

As the school year ends, Tegan and Sara get ready to play their first live show at a party. But will a secret that Tegan must keep from Sara threaten their debut performance's success? Meanwhile, Sara still feels awkward after her fight with Patrick but is excited for summer to begin so she can spend more time with Phoebe. Tegan gets closer to Cass but also misses Maya.