Songfacts®:

• Keys penned this melancholy slow-burner with Frank Ocean and his frequent collaborator, James "Malay" Ho. Their collaboration started with the pair just conversing and getting to know one another. "We were just gonna hang out and talk and next thing we knew I was like, 'but I have these chords,'" she recalled to MTV News. Malay them picked up a guitar and started strumming and according to Keys a "natural, cool vibe" emerged that became this song.
• The song is about the memory of a passed relationship during a journey "I really love ["One Thing"], because it's this story about this relationship," Keys told MTV News. "It starts out on the train and by the end of the song you figure out who I got on that train. And how there's one thing you're searching for and what happens in that story."