We know we're gonna do it right after we hear it.

"Oh, Fionna" from the episode "Adventure Time with Fionna and Cake". Original by Rebecca Sugar.
Beautiful song indeed, and Neil Patrick Harris sings it beautifully too~ I love how Fionna duets it on the end of the song...
Gotta shift the key a bit higher so my voice can catch up. And the Dulcimer. Because the original has dulcimer by Cake.

Adventure Time ©Cartoon Network/Federator Studios/Pendelton Ward