Asterix and the gladiator

When Cacofonix the bard is taken to Rome as a present for Julius Caesar, Asterix and Obelix set out to rescue him, sailing with master salesman Ekonomikrisis, the Phoenician merchant. How do our Gaulish friends come to end up training as gladiators? The next Games in the Circus Maximus are not quite what Caesar and the Roman public expect ...