The Romans capture Cacofonix as a gift from the Prefect of Gaul to Caesar. After the Gauls find this out after demolishing the nearby Roman camp Asterix and Obelix immediately set ot to Rome to rescue him.
Their first attempts are unsuccessful and they decide the only possible way to rescue Cacofonix is at the Circus Maximus, unfortunately only condemned men, lions and gladiators get into the Circus. Asterix decides that he and Obelix will become Gladiators which soon proves rather easy to arrange. Unfortunately the Gauls soon change the other Gladiators training habits until they no longer wish to fight in the arena. Caesar is not impressed with the way the Gauls have changed the Circus and sends in some crack troops to teach them a lesson. It is the troops though who are taught a lesson as the Gauls soon finish them off.
Recognising their courage (and as the crowd seemed supportive of the Gauls feat) Caesar rewards them with freedom including Cacofonix, and even the other gladiators are released from being gladiators forever more.
Asterix's next adventure is 'Asterix the Legionary'.