What is the fine-grained topic of the following text: In particular, she traces the love that develops between her modern-day heroine, Giselle Gélis, and the archaeologist David Rettig as they travel together across southern France in pursuit of a solution to the double mystery of Rennes-le-Château. Only return the topic.
Romantic relationship development between characters Giselle Gélis and David Rettig in a mystery adventure.