The purpose of AROAPOC3-3001 is to evaluate the efficacy and safety of ARO-APOC3 (plozasiran) in adult participants with familial chylomicronemia syndrome (FCS). Participants who have met all eligibility criteria will be randomized to receive 4 doses of plozasiran or matching placebo administered subcutaneously. Participants who complete the randomized period will continue in a 2-year open-label extension period where all participants will receive plozasiran.