Following the "Prelude – Light," lights go out before revealing suburban mother, Diana Goodman, alone, waiting. Gabe, her 17-year-old son, enters. Diana scolds him over his curfew, then quickly pushes him away as father, Dan, enters to check on his restless wife. Natalie, their equally restless and overachieving daughter, crosses with an exorbitant amount of homework. Soon it is morning, and each family member prepares individually for "Just Another Day." But, when Diana starts making extraneous sandwiches on the floor, the music stops. Dan helps the disoriented Diana as Natalie and Gabe exit.
Next, we find Natalie seeking refuge in a school practice room with the structure of classical piano ("Everything Else"). Her precious time preparing for an impending recital is prematurely interrupted by Henry, who introduces himself and confesses listening to her play daily. While it's clear that he likes her, Natalie pushes him out and returns to rehearsing diligently.
We then find Diana receiving prescriptions from Doctor Fine while Dan questions his own depression in the car ("Who's Crazy / My Psychopharmacologist and I"). Fine diagnoses Diana as bipolar depressive with delusional episodes. The song takes us through several weekly visits, echoed by voices selling myriad pharmaceuticals and warning of side effects. Although she's been medicated unsuccessfully for sixteen years, Doctor Fine continues to adjust her medications over the visits until Diana says that she doesn't feel anything... at which point, he declares her stable.
Over the same weeks, Natalie and Henry have grown closer over hours spent together in the practice room. We find Natalie at Henry's house as he prepares a bong. When Natalie declines smoking because she's scared of being a disaster, Henry struggles to articulate his love for her ("Perfect for You"). The song carries them to the Goodman's house, where they kiss for the first time.
Diana witnesses this event of burgeoning love, triggering memories of extreme highs and lows ("I Miss the Mountains"). With her son's encouragement, coupled with her desire to feel again, Diana flushes all of her new medications down the toilet.
We flash back to Dan, bouncing through his days ("It's Gonna Be Good"). When he discovers Henry and Natalie together on the porch, he invites Henry to stay for dinner, much to Natalie's chagrin. Dinner goes along swimmingly until Diana emerges with a cake exclaiming, "It's someone's birthday!"
Dan goes to Diana and explains "He's Not Here." Their son died 16 years ago. Devastated, Natalie storms out to her room. Henry politely excuses himself and follows Natalie. Diana confesses to Dan that she flushed all of her new medication. When Dan empathetically offers to call Doctor Fine for a new round, Diana lashes out on him ("You Don't Know"). Dan tries to calm her down and regain her trust as Gabe joins him ("I Am the One"), attempting the same.
In her room with Henry, Natalie vents about her mother. Henry attempts to placate her by making a pipe out of an apple. Natalie dismisses the offer and further explains her anger ("Superboy and the Invisible Girl"). Diana comes in and unsuccessfully appeases the situation, followed by an unseen Gabe. When Henry offers the pipe again, Natalie takes it.
Meanwhile, Dan found highly regarded Doctor Madden by networking over depression online and at work. Diana spends several weeks with Doctor Madden, who appears to her as a rock star. Gabe joins her sessions to assert his existence ("I'm Alive").
Simultaneously, Dan and Natalie are continuing to struggle with Diana's problems. Natalie, trying to cope, is lead to sample some of the her mother's pills. When she takes the stage at her recital days later, her parents' empty seats escalate her emotional turmoil. Clearly affected, Natalie verbally rejects her classical selection and launches into a sloppy rock improvisation. Henry goes to her and helps her off the stage.
Meanwhile, Doctor Madden is using hypnosis ("Make up Your Mind / Catch Me I'm Falling") to root Diana's trauma. Through this device, we learn about Diana's history with Dan, Gabe and Natalie. Finally, Diana agrees that it's time to let her son go. She goes home and begins to purge her son's things. Upon finding and opening a music box, Diana becomes drenched in a vision of dancing with her son ("I Dreamed a Dance"). When Gabe invites her to come away with him ("There's a World"), she does.
Doctor Madden enters with Diana's hospital chart. Diana Goodman was discovered in her home unconscious with self-inflicted razor wounds. Diana is treated, sedated and restrained. Doctor Madden is upset.
Dan joins Doctor Madden at the hospital. Madden explains that ECT (electroconvulsive therapy) is the next standard course of treatment for drug-resistant patients who are imminently suicidal. However, the therapy requires both Dan and Diana's consent. Dan goes home to clean up, sort his thoughts and decide what to do ("I've Been").
The next day, Diana (with Gabe at her side) receives Doctor Madden's suggestion. She lashes out at Doctor Madden, refusing the treatment ("Didn't I See This Movie?"), until Dan arrives and convinces her that it may be their last hope ("A Light in the Dark"). Dan and Gabe watch as she signs the consent form and is ushered out, briefly pausing to look back.
While Diana begins a two-week series of ECT treatments, we find Natalie losing her discipline and further experimenting with drugs and clubs ("Wish I Were Here"). Natalie is home when Dan brings Diana home from the hospital. Diana doesn't recognize her daughter. The Goodman family realizes that Diana has lost 19 years of memories ("Song of Forgetting").
Natalie leaves for school, where Henry confronts her ("Hey #1"). He questions her avoidance, sobriety and commitment, as well as asks her to the spring formal dance. She leaves him without an answer.
Dan and Diana visit Doctor Madden, who assures them that Diana's memory loss may be partly psychogenic ("Seconds and Years"). Doctor Madden encourages Dan to use pictures and keepsakes to trigger memories and help Diana recover. Dan and Natalie gather to do so ("Better than Before") with minor success. When the music box emerges from the pile of keepsakes, Dan whisks it away, leaving Diana puzzled. Gabe appears, unseen and unheard, as he sings about which is worse – the symptom or the cure ("Aftershocks").
Diana continues searching the mementoes for days, trying to uncover a memory that's tugging at her. When Henry arrives looking for Natalie, Diana is given great pause, studying his face and asking his age. Not phased, Henry hurries up to Natalie's bedroom to convince her to go to the dance. ("Hey #2"). When Natalie avoids accepting, Henry promises to try again the following evening. He leaves her with a ticket to attend.
Diana returns to Doctor Madden ("You Don't Know – Reprise") by herself. He nearly discloses the root of her trauma (which is Gabe's death) before suggesting that she further explore her history and talk with her husband. Diana goes home and finds the music box. Dan tries to stop her, but the irrevocable memories of her baby boy rush back ("How Could I Ever Forget?"). When Diana confesses remembering her son as being older, Dan quickly insists they return to Doctor Madden ("It's Gonna Be Good – Reprise"). At the same time, Henry arrives to pick up Natalie for the formal. Dressed for the dance, Natalie emerges in time for both to witness Dan grab and smash the music box to pieces on the floor. Natalie is crushed and runs to her room. Henry follows.
Diana confronts Dan, wondering why he perseveres after how much trouble she's been. Upstairs, Natalie echoes her mother's sentiments to Henry ("Why Stay?"). Dan and Henry both answer with vows to stay steadfast and true ("A Promise"). Natalie and Henry embrace as Gabe reappears downstairs ("I'm Alive – Reprise"). Seeing him, Diana bolts to see Doctor Madden. Natalie agrees to drive her and tells Henry that she will try to meet him later. Gabe lingers with Dan.
Diana asks Doctor Madden what can be done now. Madden assures her that relapse is common. Diana discovers that it's not her brain that's hurting; it's her soul ("The Break"). Doctor Madden urges her to continue ECT treatment ("Make up Your Mind / Catch Me I'm Falling – Reprise"). If left untreated, her chronic illness could be catastrophic. Diana declines further treatments and walks out.
Diana explains her decision ("Maybe – Next to Normal") to Natalie outside, opening up to her daughter for the first time. She urges Natalie to go see Henry at the school dance. Natalie arrives and finds Henry standing alone ("Hey #3 / Perfect for You – Reprise"). Immediately struck by her beauty, Henry then calms Natalie's fears that she might end up crazy. They embrace and kiss.
At the same time, Diana finds Dan at home and tells him that she's leaving him ("So Anyway"). She explains that he can't always be there to catch her. She needs to take a chance and deal with things on her own. She departs, leaving Gabe with Dan.
As Dan ponders how she could have left him ("I Am the One – Reprise"), Gabe slowly approaches. Dan grows increasingly distraught until he, at last, faces the boy and calls him by his name for the first time: Gabriel.
Natalie returns home to find her father, sitting alone in the dark, in tears. She comforts him, assures him that they will be okay and begins to turn the lights on in the house ("Light"). We come across Diana, alone and still hurting, yet happy to be alive. We see Henry and Natalie the next day, committed, come what may. We view Dan visiting Doctor Madden. Although he entered to get an update on Diana, he stays to talk about his own struggle. Finally, we see Gabriel arrive, watching over them all. Normal life continues.