After quelling the coup d'état that shook the Kingdom of Liberl, Estelle and Joshua have finally become senior bracers. However, on the eve of the Queen's Birthday Festival, Joshua reveals the dark truth of his past before vanishing into the night, leaving only his brief farewell: "Goodbye, Estelle." Armed with Joshua's harmonica and an unshakable determination, Estelle embarks upon a continent-spanning quest to find her partner. During her journey, she must take on the sinister society of Ouroboros as it emerges from the shadows to plunge Liberl into utter chaos. The capital in flames. A colossal airship darkens the skies. The looming threat of the Shining Ring... Confronted with the greatest crisis in the history of Liberl, Estelle and her allies battle to find Joshua and untangle the web of intrigue and conspiracy woven by Ouroboros!