Flo and his trusted Collie pal Lassie spend the summer at Aunt Cosma's, alongside her foster children Kleo and Henry, and her dog Pippa. When local canines start disappearing, including Pippa, Lassie rallies the kids to solve the mystery.

12-year-old Florian and his beloved dog Lassie live in an idyllic village in the German countryside. When Florian's father loses his job, the family have to move into a small apartment where there are no dogs allowed. Lassie is given away to another family, but Florian and Lassie yearn to be reunited again so Florian sets out on an adventure to find his best friend.