Here's what happens in the flashback:
* Christine's father is dying, leaving her orphaned.
* The Phantom appears and promises to take care of Christine, claiming that he's an angel sent by her father.
* Christine is scared and confused but ultimately trusts him.
* The Phantom leads her away from the chaos of her father's death, promising to teach her music and be her guardian.
This flashback is crucial in understanding the Phantom's motivations and Christine's vulnerability. It sets the stage for their complex relationship and the Phantom's obsessive desire to protect and possess her.