After meeting her real mother, Irene, upset by the changes in her life, finds refuge in the friendship that binds her with Sherlock and Arsène. At the beginning of summer, following the girl's invitation, the three inseparable friends meet in Davos against the backdrop of the magnificent Swiss Alps. However, behind the serene comings and goings of travelers lies an international conspiracy. In an atmosphere of suspicion, where no one is what they seem, Irene and her friends get entangled in the web of a great criminal and with a daring mission to the formidable castle that dominates the valley, they will force him to reveal his plans.