"The Adventure of the Beryl Coronet" follows Alexander Holder, who entrusts a valuable coronet to his care. The coronet is stolen, and Holder's son is suspected. Holmes investigates, uncovering the son's misguided attempt to impress a woman by borrowing the coronet. However, her unscrupulous fiancé damages it while attempting to steal the gems. Holmes retrieves most of the missing stones but exposes the fiancé's deceitful nature. The story concludes with partial recovery of the coronet and lessons about trust, family, and the consequences of misplaced trust in pursuit of affection.