This study is researching an experimental drug called garetosmab. The study is focused on adult patients with fibrodysplasia ossificans progressiva (FOP). The aim of the study is to see how safe and effective the study drug is in patients with FOP. The study is looking at several other research questions, including: * What side effects may happen from receiving the study drug * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)