Mark Antony, Octavius Caesar and Lepidus, having defeated Julius Caesar's assassins at Philippi, now rule the Roman Empire as a triumvirate.
While in Alexandria however, the ageing Antony has become captivated by Cleopatra, Queen of Egypt (and mother to Julius Caesar's illegitimate son, Caesarion). The gossip and scandal this is creating both amongst Romans in Alexandria and at home in Rome gives rise to dissention between Octavius and Antony, whose behaviour is felt to be debauched and 'un-Roman'.
At the same time as the power of the triumvirate is being challenged by a dissatisfied senator, Pompey, Antony hears news from Rome that his wife, Fulvia, is dead. These two issues together force Antony to return to Rome and take up his responsibilities as a triumvir again.
Once back in Rome, Antony seems less controlled by his fascination for Cleopatra and, in an attempt to strengthen the triumvirate and cement his political alliance with Octavius following a quarrel, he agrees to marry Octavius's sister, Octavia. This news drives Cleopatra into a jealous rage.
On the brink of another bloody civil war against Pompey's forces, Antony and Octavius manage to negotiate a peace and they, along with Lepidus, feast with Pompey in celebration.
Antony and Octavia then leave for Athens, where Antony has been summoned to quell a rebellion by the Scythians. No sooner have they arrived there than Antony learns that Octavius has ignored the agreed peace treaty, has taken arms against Pompey once more, is plotting against Lepidus, the third member of the triumvirate, and has also spoken critically of Mark Antony. Enraged, Antony sends Octavia back to Rome to act as a go-between but also prepares for war against Octavius.
Octavius learns that Antony has returned to Alexandria and, with Cleopatra, has appeared enthroned in the market place, crowning themselves and their children as kings and queens. Octavius declares war on Egypt and, despite warnings not to fight at sea, Antony agrees that the two navies will meet for a sea battle at Actium.
The Egyptians, under Antony's command, lose when he deserts the battle to follow Cleopatra's fleeing ships. Antony is ashamed and in despair at his own unsoldierly behaviour. But when he hears that Octavius is planning a secret peace with Cleopatra at the expense of Antony's own life, he has Caesar's messenger whipped and rouses himself for a second battle in which he is victorious.
Before the third and decisive battle, many of Antony's soldiers desert him fearing bad omens, including his most loyal friend Enobarbus. A disappointed Antony sends after Enobarbus all the treasures he had left behind on his desertion, and Enobarbus is so stricken with shame that he dies.
Having won the initial battle by land, Antony prepares to face Octavius's forces again at sea.
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The Egyptian navy deserts, leading the defeated Antony to believe that Cleopatra has betrayed him to Octavius. She is so angry that she retreats to her monument and sends false word to Antony that she has committed suicide.
Appalled, and echoing the suicide of the conspirator Brutus at Philippi, Antony begs a faithful servant to hold his sword while he falls upon it. Unwilling to do so, the servant, Eros, kills himself. Antony then attempts suicide but fails, leaving himself badly wounded.
A messenger arrives from Cleopatra, telling Antony of her deception. Antony instructs his guards to take him to Cleopatra's monument where he is raised up to the top of the monument to die in her arms.
Having persuaded Octavius that she will surrender, but fearful of capture and the shame of being exhibited as a defeated enemy through the streets of Rome, Cleopatra holds a poisonous snake to her breast and dies, along with her faithful maids.