A Study to Assess Safety, Tolerability and Efficacy of Garetosmab Versus Placebo Administered Intravenously (IV) in Adult Participants With Fibrodysplasia Ossificans Progressiva (FOP)

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)

Key Inclusion Criteria: 1. Clinical diagnosis of Fibrodysplasia Ossificans Progressiva (FOP) \[(based on findings of congenital malformation of the great toes, episodic soft tissue swelling, and/or progressive Heterotopic Ossification (HO)\] 2. Confirmation of FOP diagnosis with documentation of Type I activin A receptor (ACVR1) FOP causing mutation 3. FOP disease activity within 1 year of screening visit. FOP disease activity is defined as pain, swelling, stiffness, or other signs and symptoms associated with FOP flare-ups; or worsening of joint function, or radiographic progression of HO lesions (increase in size or number of HO lesions) with/without being associated with flare-up episodes 4. Willing and able to undergo CT imaging procedures and other procedures as defined in the protocol Key Exclusion Criteria: 1. Cumulative Analog Joint Involvement Scale (CAJIS) score at screening \>19 2. Participant has significant concomitant illness or history of significant illness such as but not limited to cardiac, renal, rheumatologic, neurologic, psychiatric, endocrine, metabolic, or lymphatic disease, that in the opinion of the study investigator might confound the results of the study or pose additional risk to the patient by their participation in the study 3. Previous history or diagnosis of cancer 4. Severely impaired renal function defined as estimated glomerular filtration rate \<30 milliliter per minute (mL/min) (/1.73 m\^2 calculated by the Modification of Diet in Renal Disease equation 5. Uncontrolled diabetes defined as hemoglobin A1C (HbA1c) \>9% at screening 6. History of poorly controlled hypertension, as defined by: 1. Systolic blood pressure ≥180 mm Hg or diastolic blood pressure ≥110 mm Hg at the screening visit 2. Systolic blood pressure of 160 mm Hg to 179 mm Hg or diastolic blood pressure of 100 mm Hg to 10\^9 mm Hg at the screening visit, AND a history of end-organ damage (including history of left-ventricular hypertrophy, heart failure, angina, myocardial infarction, stroke, transient ischemic attack, peripheral arterial disease, end-stage renal disease, and moderate-to-advanced retinopathy 7. Known history of cerebral vascular malformation 8. Cardiovascular conditions such as New York Heart Association class III or IV heart failure, cardiomyopathy, intermittent claudication, myocardial infarction, or acute coronary syndrome within 6 months prior to screening; symptomatic ventricular cardiac arrhythmia 9. History of severe respiratory compromise requiring oxygen, respiratory support (eg, bilevel positive airway pressure \[biPAP\] or continuous positive airway pressure \[CPAP\]), or a history of aspiration pneumonia requiring hospitalization 10. Prior use in the past year and concomitant use of bisphosphonates 11. Concurrent participation in another interventional clinical study or a non-interventional study with radiographic measures or invasive procedures (eg, collection of blood or tissue samples) 12. Treatment with another investigational drug, denosumab, imatinib or isotretinoin in the last 30 days or within 5 half-lives of the investigational drug, whichever is longer 13. Pregnant or breastfeeding women 14. Women of childbearing potential (WOCBP) who are unwilling to practice highly effective contraception, as defined in the protocol 15. Male patients with WOCBP partners who are not willing to use condoms with WOCBP partners to prevent potential fetal exposure, as defined in the protocol Note: Other protocol defined Inclusion/Exclusion Criteria apply