The author reconsiders various interpretations of the enigmatic story told by Pausanias several times (II. 8. 5, VII. 7. 3; VIII. 10. 5–10; 27. 13–14; 36. 5–6). The main character of the story is the Spartan king Agis, son of Eudamidas (thus, Agis III), who undertakes an unsuccessful war against the Achaean League and the Arcadians, attacks Pellene, tries to assault Megalopolis and finally dies in the battle of Mantinea. Modern scholars try to find the historical basis of this account full of contradictions and anachronisms. In the author’s opinion, the whole of the story of Agis’ expeditions and defeats goes back to an unauthentic oral tradition, which originated in Mantinea among the descendants of Achaean colonists who inhabited the city after its population had been sold into slavery in the late 20s of the 3rd c. BC. New inhabitants of the city, poorly informed of its real history, created a legend about brilliant victories won by the Achaeans together with their allies Arcadians (including Mantineans) over Sparta under Agis III, though the historical reality was quite different.