You must complete Ria, Born to Roam (I) in order to gain access to this Side Story in Wellspring. Ria’s journey has brought her to Wellspring, but while staying there, a thief broke into the place she was staying at and stole something very important to her. On the western side of town, there is a Traveling Merchant, who has the letter of Ria’s. Using Therion or Tressa, Steal/Purchase the Tightly Sealed Envelope from the merchant, then bring it back to Ria to finish the quest.