The song 'The One That Got Away' by Katy Perry is a nostalgic reflection on a past romantic relationship that ended before its time. The key themes include regret, lost love, and the passage of time. The lyrics vividly recount moments and memories shared with a past lover, expressing sorrow over how things ended and the enduring feeling of 'what could have been'. Particularly poignant moments in the song involve the imagery of them making promises and dreaming about the future, contrasted with the heartbreak of realizing those dreams will never come true. Ultimately, the song captures the universal experience of longing for a love that's been lost.