The story is set in the late 5th century, during the fall of the Western Roman Empire. The plot centers around the young Roman Emperor Romulus Augustulus (played by Thomas Sangster), who is deposed by the barbarian chieftain Odoacer (played by Peter Mullan). As the empire crumbles, Romulus escapes with his loyal protector, Aurelius (played by Colin Firth), and a small band of soldiers.

The group seeks refuge with the legendary Last Legion, a secretive group of elite soldiers who have sworn to protect the Roman legacy. They are joined by a skilled warrior woman, Mira (played by Aishwarya Rai), who helps them on their quest. Their journey leads them to Britain, where they hope to find the fabled sword of Julius Caesar, Excalibur, which they believe will help them restore the empire and combat the forces of the barbarian invaders.

As they travel, they face numerous challenges and battles, including confrontations with treacherous enemies and internal conflicts within their own ranks. The film features epic battle sequences, sword fights, and a quest for a legendary artifact that ties the characters to the mythic past of Rome.