Tartini's Key.

The action takes place in Piran, where three children meet. An SMS message was sent to the wrong number triggers a series of events in which children go through various adventures and puzzles related to Piran. The antagonists of the story are bandits who are looking for treasure, but they are overtaken by children who are more skilled in solving the keys that lead to the treasure. The children are the first to find the treasure, which is actually a collection of letters between the famous violinist Tartini and Stradivari, a violin maker who through correspondence provided him with support and advice on improving violins.