ㄴ가 설레다

somehow excited, a flutter of anticipation

Every line here is 뭔가 "somehow, something" in front of 설레다, the feeling of your heart fluttering with anticipation. 뭔가 is doing a lot of work: the speaker can't name the reason, so it is "I'm kind of excited", "there was something exciting about it", "a drama that gives you that flutter". 설레다 is forward-looking excitement about something to come, not general happiness, and it is a soft, slightly self-conscious word.

Register: casual, soft