Mr. Moonlight by The Beatles is a song that revolves around the theme of longing and romantic devotion. The narrator expresses heartfelt emotions towards a significant other, singing about the impact of their presence under the moonlight. The key themes include romantic love, yearning, and the beauty of shared moments beneath the night sky. One poignant moment in the song is when the narrator emphasizes how the presence of their loved one brightens their life, signifying deep emotional connection and attachment.