More than anything, Breathless was about the vibe it created than the story it told. It was vaguely about a guy who killed a cop and now was trying to lay low, but most of the movie is him and a girl forming a relationship, chatting about nothing. It's perfectly relaxing.
There's a 15-minute scene of them just chatting in a hotel room. Just a clip of it sums up what the whole movie was like.
Every scene of this movie felt like a breezy summer day. I could leave it on loop all day.
As a bit of a bonus, my favorite YouTube channel, The Nerdwriter, did a piece on how World War II changed cinema through the lens of Breathless.