[3] The film opens during Caesar's invasions of Britain, with Mark Antony (Sid James) struggling to lead his armies through miserable weather.

At a nearby village, cavemen Horsa (Jim Dale) and Hengist Pod (Kenneth Connor) attempt to alert Boudica to the invasion, but are captured by the Romans.

Once in Rome, Horsa is sold by the slave-trading firm Marcus et Spencius, and Hengist is destined to be thrown to the lions when no-one agrees to buy him.

Whilst hiding there, Julius Caesar (Kenneth Williams) arrives to consult the Vestal Virgins, but an attempt is made on his life by his bodyguard, Bilius (David Davenport).

When a power struggle emerges in Egypt, Mark Antony is sent to force Cleopatra (Amanda Barrie) to abdicate in favour of Ptolemy.

He and Caesar then ally with Horsa, and after defeating Cleopatra's bodyguard Sosages (Tom Clegg) in combat, Hengist and the party flee Egypt.

Horsa and Hengist return to Britain, and Mark Antony is left in Egypt to live "one long Saturday night" with Cleopatra.