Kara and Barry are from 2 different universes and after the break up with Iris the two superheros meet up and eventually start to develop feelings for each other but what they didn't realize until they got their bond that they lived in the same world and the lightning bolt was the reason they forgot each other.

Based On The Cw Arrowverse Crossovers Elseworlds and Crisis On Earth X "Why were you so afraid to tell me you were falling in love with me?!" " I wasn't just afraid to tell you, I was afraid to fall knowing that you weren't gonna catch me"