This mystery can be found in the northeast of Wincestre, just south of the Trader. You can begin the mystery in two ways: by speaking with Esobert on the scaffolding of the church to the east first, or taking the flower from the tree first. Either way, you’ll need to take the flower from the tree by climbing it from the ground, or jumping to it from the rope wire near Esobert.
Once you have the flower for Esobert, jump down from the tree and speak with him. A cutscene will play during which Esobert presents the flower to princess Aethelflaed. At the end of the scene, the mystery will complete.