I'm not sure without reading the story, but it could be a protagonist who is haunted by the 'one that got away' and perhaps a secondary character who was part of the situation that led to the loss.
There might be a narrator who is looking back on their life and recalling the person or thing that got away. And there could be another character, like a friend or a love interest, who was directly involved in the event that made this person or thing become 'the one that got away'. For example, if it's a love story, the love interest could be the main character that the narrator lost, and a mutual friend could also be part of the story, adding to the complexity of the situation.