Will is a 29-year-old paralegal working at a prestigious law firm in NYC. He used to be a musician, but left his art behind for law school and stability. One day, he meets Alex, a new co-worker with a similar path to his- she left behind a career as a singer-songwriter. They quickly become friends.

 

Inside Will, his Heart, Nervous System, Ego, and Reproductive System- also a Rock Band that holds onto the music their human host left behind ("IN THIS BODY")- are excited about the prospect of a new friend. The Heart also confesses romantic feelings for Alex, but is chastised by the others who cite the mysterious Power above- whose orders they obey- and its de-prioritization of Love. After an emotional plea from the Heart ("ANOTHERHEART"), the Power seems to reverse course as Will suddenly falls in love with Alex, but after a month of hiding his feelings, she tells him she’s seeing someone. Will, and the Band, are heartbroken. 

 

Two months later, the Band comes to grips with the fact that Will isn’t over Alex, and that having to hide his unrequited love is making him depressed. The Ego suggests that the Power is wrong to let his feelings linger, but the Heart, dead set on Love, wants to follow them. As they debate, they guide a drunk Will through the firm’s holiday party, where he reveals to Alex that his parents “disappeared” when he was young- a memory the Band can’t recall.

 

As Will’s depression accelerates, the Nervous System journeys to the Power alone to confront it in secret, but the Heart stops her. The Band edges closer to the brink of civil war, when Alex invites Will to her apartment. She’s struggling too, and wants to talk. 

 

Alex tells Will that she wants her old life back as an artist, and suspects Will feels the same way. After playing a song of hers ("FALLING UP"), Will confesses his love, breaking Alex’s heart as he has been her only friend at work. Will’s heart is broken too. He leaves, and breaks down.

 

The Heart is finally convinced to join the rest of the Band in confronting the Power to save Will ("THE ROAD"). The Power takes human form- Will himself- as the Band finally remembers…The Brain. They also remember and re-experience his parents’ divorce and their resulting lack of care for him, learning that Love was taught to Will as being met halfway, explaining why he fell in love with someone who doesn’t love him back. They realize that the Brain is an organ just like them, and only when they work together can Will be happy. Reunited with its Band at last, the Brain can finally sing its song.

 

Will starts to recover. Alex decides to quit and go back to music, and they part friends. The Band celebrates their newfound friendship with the Brain, and all the possibilities that lay ahead as Will starts to write his first song in years.