Opus centers on a world-renowned string quartet struggling to prepare for their highest-profile performance when the violist, and founder of the quartet, mysteriously disappears. When a young woman is hired as the replacement, her musical gifts inspire the musicians, and they decide on short notice to abandon their Pachelbel Canon program and to replace it with Beethoven's difficult Opus 131. The rehearsal room becomes a pressure cooker as passions rise, personalities clash and the musicians are forced to tackle the ephemeral nature of their life's work.