Paul McCartney's 'Once Upon A Long Ago' is a nostalgic ballad that delves into cherished memories of the past. The song evokes a sense of longing and reflection on moments that have slipped away, capturing the bittersweet nature of reminiscing. Key themes include nostalgia, memory, and the passage of time. Particularly poignant moments in the song include the way McCartney describes scenes and incidents with a wistful tone, highlighting the emotional weight of recollections.