In one night, Natalie brings Henry to three different nightclubs, which concerns Henry. He suspects that Natalie is using drugs, which she admits to, listing Diana’s medication drugs. She also tries to justify that she is using them to relieve the stress of Diana undergoing electroconvulsive therapy. 

At the hospital, Doctor Madden initiates the procedure by sedating Diana with anesthesia. Diana has an out-of-body experience and watches herself undergo treatment, commenting that she doesn’t feel present in herself as she achieves mental clarity (“Wish I Were Here”). Parallel to Diana, Natalie comments that she is experiencing exuberance because of the drugs, but cannot feel any of it. They register each other in a liminal space outside of their respective bodies. Diana cautions her against using drugs; Natalie accuses her of being hypocritical. They reiterate their shared lack of feeling, as well as their desire to feel. Natalie collapses in the club and Henry comes to her rescue.

Dan picks Diana up after her treatment ends. Natalie and Henry make it back home before Natalie’s parents do, but she urges Henry to leave at once. She changes her clothes before her parents can see her. Diana registers Natalie’s presence, but cannot remember who she is. It occurs to Dan and Natalie that Diana has lost her memories of her family life, spanning the past 19 years. Dan tries to restore Diana’s memories by walking her through the house (“Song of Forgetting”). All these efforts fail. Natalie turns cynical, indicating that the treatment made Diana’s memories irretrievable. 

At school, Henry greets Natalie, whom he hasn’t seen in some time (“Hey #1”). Henry expresses concern over Natalie’s drug use, which Natalie criticizes as hypocritical. Henry remains committed to their relationship, but feels that the girl he fell in love with is repressed in Natalie. He invites her to go with him to the spring formal dance. Natalie declines, prompting Henry to chase after her.

At the clinic, Doctor Madden reminds Dan that Diana’s memory loss was always a possibility, just like the chances of those memories being recovered. Dan complains about living with Diana’s memory loss for two weeks already (“Seconds and Years”). Doctor Madden responds by getting Diana to express her feelings of clarity and lightness. He then advises Dan to look for keepsakes that can help to trigger Diana’s memory recovery, but cautions him to refrain from reminding her about Gabe for the time being (“Better Than Before”). 

Dan starts by showing Diana keepsakes from their wedding, though Natalie protests that Dan is misrepresenting their elopement as something idyllic. When Dan moves on to photos from their early family life, Diana fails to recognize Natalie once more, triggering Natalie’s frustration. Dan reassures her that things could go back to normal if they try their best to steer Diana’s memories of the past. 

Natalie shows Diana keepsakes of incidents that hint at bad memories, such as their house catching fire and Diana crashing the car while giving Natalie a parking lesson. To Natalie’s surprise, Diana recalls an incident where she had missed Natalie’s recital because her excessive lithium medication caused her to hide in the car. Diana acknowledges that Natalie has lived through a hard childhood, which causes Natalie and Dan to react with joy and the optimism that things will improve. However, Diana stumbles upon the music box, which sparks a brief recollection of Gabe.

The vision of Gabe comments that while Diana has cleared away the memories that affect her, she will continue to feel the grief of losing him (“Aftershocks”). Diana begins rifling through her belongings to look for the thing she feels is missing in her life. Dan reassures her that whatever she is trying to remember will come back to her eventually. 

One day, Diana answers the door when Henry comes looking for Natalie. Diana asks how old Henry is, remembering someone around his age. Henry reminds Natalie that the spring formal is taking place the following day (“Hey #2”). Despite Henry’s insistence on resuming their relationship, Natalie stresses that their relationship triggers her worst instincts. Henry makes a final appeal to restart their relationship on a clean slate. He promises to pass by to pick her up for the dance, but if she doesn’t come, he will accept her decision.

On the fourth week after her electroconvulsive therapy treatment, Diana continues to wonder what she is trying to recover from her lost memories (“You Don’t Know (Reprise)”). She shares this anxiety with Doctor Madden, and acknowledges that most of the memories she has recovered have been mediated by Dan. When Doctor Madden mentions Diana’s son, Diana responds with shock, prompting Madden to realize his error. He advises Diana to talk to Dan and abruptly ends their session.

A vision of Gabe hands the music box back to Diana. Dan sees Diana holding the music box and asks her where she got it. Diana begins to remember they used to play it for their son to put him to sleep. Soon, Diana remembers the day Gabe died (“How Could I Ever Forget?”). Dan discourages her from remembering everything about that day, telling her it won’t help her mental health. Eventually, he starts recalling that day as well, remembering it as the day he lost the Diana he knew. Diana asks Dan to explain what happened because she wants to remember everything, including the things that hurt her: The baby was experiencing a slew of symptoms that the doctors misinterpreted as common infancy issues. These symptoms pointed to an underlying illness that caused the baby’s sudden death. 

Diana struggles to remember the baby’s name. Dan tries to placate her with promises to return to Doctor Madden (“It’s Gonna Be Good (Reprise)”). Diana insists on recalling her son’s name while Henry arrives to pick up Natalie. Natalie comes downstairs, fully dressed for the formal. She sees Dan take the music box and smash it to the ground. This causes Natalie to retreat back to her room. Henry follows her upstairs. Diana asks why Dan chooses to stay with her when their relationship negatively affects his life (“Why Stay?”). Upstairs, Natalie asks Henry a similar question, suggesting that he shouldn’t have to endure her problems.