In Ancient Britain, Hengist Pod makes a poor living as the maker of square wheels, much to the dismay of his shrewish wife Senna. He sells one of his wheels to Horsa, his new next-door neighbour, and the two become friends. Their settlement comes under attack from a Roman legion and Hengist is sent to get help from Boudicca's army in Carlisle. After his bicycle breaks, he tries to get a lift from a passing cart, but is captured by the Romans. Together with Horsa and others he is sent to Rome.

Mark Antony, the leader of the invading Roman army, goes to see Julius Caesar and update him on their progress. They are both unhappy about their lack of success and the bad weather. Caesar is recalled to Rome after three years away at war with news of possible treachery in the Senate. The population greets Caesar's return with disdain, as does his wife Calpurnia, who feels ignored by her husband. He brings her the gift of a British slave, Gloria, but she rejects the offer. Her father Seneca, however, is much more taken with her. Seneca warns Caesar to beware the ides of March and of plots against his life.

Horsa and Hengist are taken to the slave market run by the brothers Marcus and Spencius. After Horsa is sold at auction, the two manage to escape and hide out in the Temple of Vesta. Caesar arrives to consult the Virgins. Shortly after arriving, Caesar's bodyguard, Bilius, tries to kill him. Caesar and Hengist are knocked senseless and Horsa dispatches Bilius and the guards on his own. He manages to hide just before Mark Antony and his men arrive. Hengist is given the credit for saving Caesar's life and is made a centurion, replacing Bilus as Caesar's bodyguard. Mark Antony, however, remains unconvinced of his fighting skills.

Mark Antony is sent to Egypt to depose Cleopatra and return Ptolemy to the throne. However, he is soon bewitched by her charms and they quickly become lovers. He kills Ptolemy and leaves her on the throne. He plots with Cleopatra to overthrow Caesar and become the new Emperor. Mark Antony returns to Rome and tells Caesar that Cleopatra wishes to have a liaison with him. Caesar heads for Egypt accompanied by Mark Antony, Hengist, Seneca and Gloria. Horsa has been recaptured and is one of the galley slaves manning the oars on the ship. He and the others escape from their shackles and overwhelm the guards on the ship. Mark Antony has arranged for the ship's men to kill Caesar and Hengist is sent to fight them. He finds that Horsa and the other slaves have killed the rebels and have left the ship. Once again Hengist is given all the credit. Those remaining on board now have to row the ship all the way to Egypt.

Once the Romans have arrived in Alexandria, a soothsayer tells Caesar that Cleopatra will try to kill him when he goes to her chamber than night. Hengist and Caesar exchange clothes, unaware that Mark Antony is hiding under the bed waiting to stab Caesar. Hengist is so nervous about impersonating Caesar that he is given a potion that gives him great strength and courage. When he meets Cleopatra he jumps on her bed and inadvertently immobilises Mark Antony. He then fights Cleopatra's bodyguard, Sosages, before meeting up with Horsa. Together with Gloria they escape and return to Britain. Horsa and Gloria are married and, thanks to the potion, Hengist and his wife go on to have several children. Brutus and other members of the Roman Senate eventually assassinate Caesar, while Mark Antony stays on in Egypt to live with Cleopatra.