Warning: All spoilers are unmarked.
OPAL begins with the title character, a little girl, surrounded by her loving family. They sing a song about how they see her and love her, then present her with a hamburger. Instead of eating it, she does a strange dance, which her family approves of.
Suddenly, Opal hears someone crying in the house across the street. She makes her way over to it, but is hesitant to go in - until a bright flash panics her and she rushes inside out of instinct.
Inside the house, Opal soon encounters an old, blind man sitting in front of a television. The man doesn't recognize her, referring to her as "Claire" and asking her to retrieve his cigarettes. The old man (Claire's grandfather) laments that the people on television have it so much easier, which leads into a song ("Easy to Breathe") about the ideal life the characters on TV have. When Opal doesn't response, Claire's grandfather becomes suspicious and lunges out of his chair, chasing after her. Opal, naturally terrified, retreats up the stairs.
As she passes an open door, a man shouts for her to come in and at least say hello. This is Claire's father, a narcissist surrounded by mirrors, who speaks to Opal in a condescending tone, putting her down in order to improve his own self-image. He sings a song ("Mirror Man") about how beautiful he is, and how much work he has put in to remain this beautiful. At the end of the father's song, Claire's grandfather catches up to them and she retreats out of fear.
Before she can get far, a violent Addled Addict snatches her by the ankle. This is Claire's mother, who relies on her daughter to care for her when it should be the other way around. In her drug-and-alcohol-induced haze, she sings a song ("Virtuous Cycle") about the Vicious Cycle that is her life. Opal wrenches her ankle free and breaks away into the attic, closing and locking the door behind her (and shoving a chair under the doorknob for good measure).
Finally safe (for the time being; Claire's family can be heard beating on the door), Opal sees a small loft assembled in the attic... and a window showing a billboard for "Opal's Burgers". This is The Reveal - Opal is Claire. She imagines herself surrounded by a loving family in order to get away from the nightmarish, dysfunctional life she's trapped in.
With a scream, Claire retreats once again to that fantasy as the short ends.