Some hours later, Panacea receives word that her fiancé, Tragicomix, has been forcibly-conscripted into the Roman army and shipped to North Africa; and Obelix, although heartbroken, promises to bring him back.

Asterix and Obelix travel to Condatum, where they learn that Tragicomix has already left for Massilia, the Mediterranean port from which the soldiers depart, and enlist themselves in the army to follow him, alongside Hemispheric the Goth; Selectivemploymentax the Briton; Gastronomix the Belgian; Neveratalos the Greek; and Ptenisnet, an Egyptian tourist who spends the entire book believing himself to be in a holiday camp.

After completing basic training (and repeatedly and comically driving their instructors to the verge of tears), the newly formed unit sets off as reinforcements to Caesar against Scipio, Afranius, and King Juba I of Numidia.

Asterix and Obelix soon find out that Tragicomix has gone missing in action after a skirmish, and raid Scipio's camp to recover him.

This results in the Battle of Thapsus, in which the confusion over the Gauls' unorthodox assault and the similarity of both armies' uniforms cause a default victory for Caesar after the frustrated Scipio sounds the retreat.