This idea came to me when I was singing The Greatest Showman nonstop today, so...... Yeah

-person one (PI)Xperson two (PII)
-PII is singing songs from <a musical> and their balcony window is open
-PI is walking past and hears
-PII is singing PI's favourite musical
-they both get sucked into that musical somehow
-when there they have parts to play
-they have to play along with it
-they have to try not to change the musical
-they fall in love/get together (duh) (kinda duh)

That's the general idea.......