Toward the end of his reign, Roman Emperor Marcus Aurelius (Alec Guinness) intends to hand over his throne to the steadfast soldier Livius (Stephen Boyd). Before he can enact his intentions, a loyalist to Commodus (Christopher Plummer), the de facto heir, poisons Aurelius. Commodus becomes an ineffectual leader, corrupt and pompous -- and although Livius concedes to Commodus' rule, he cannot stand by when Commodus threatens to kill Aurelius' daughter, Lucilla (Sophia Loren).