Conni, the shamrock and the horses by the sea

Conni, Anna, Billi, and Dina spend a week at a riding stable by the North Sea. The farm is beautiful, the owner is nice, and the weather is just as splendid as the horses. Everything is just as the four friends had imagined. They receive riding lessons and take long rides to the beach. But then the girls suddenly find themselves involved in a real horse mystery.