Lyon Gaultier works in the French Foreign Legion. His brother dies a traumatic death when a deal went wrong. Lyon abandons the legion and escapes. He boards a steamer which is heading towards the USA. In the meantime, two legionnaires set out to find out Lyon. Lyon arrives in California. On his way he comes across Joshua who is a professional fight organizer. Through Joshua he meets Cynthia, who agrees to finance him for a fight. Lyon decides to participate in a fight and earn some money for his family. He finally tracks down his brother's widow in LA.
Joshua introduces Lyon as "Lionheart" to all the fights he takes part in. Lyon continues to participate in the fights and earns more money.
Meanwhile, the two Legionnaires find Lyon in LA. Lyon manages to escape from them but while doing so he suffers from broken ribs. Cynthia decides to take advantage of Lyon's accident and she arranges for a fight with a great champion fighter "Attila the Hun". She bets a huge sum of money on "Attilla". Lyon's defeat is quite evident and thus, after his defeat she promises to hand Lyon to the Legionnaires.
The mammoth fight begins and Lyon has to pay the price for his broken ribs. Lyon seems to be very fragile before the giant strength of "Attilla", seeing this Joshua too bets all his money on "Attilla". Lyon is ardently warned by Joshua to give up. But, at last Lyon gathers all his remaining strength and manages to defeat "Attilla". The Legionnaires arrive to bring Lyon back to Africa where he will be having his court martial. He is allowed to bid his last goodbyes to his family. Along the way, the Legionnaires release Lyon out of deep respect for his immense courage and generosity towards his family and his brother's widow.