As a(n Earth Pony) direct descendent of the titular Unicorn of this volume, I must say it's by far my favorite.
The story starts off differently than the rest in the series; we're following Bravado in his continuing quest for "the fragments", having already procured three of the seven, and looking for the fourth down in Brayzil. He crosses paths with Daring along the way (as he usually does), but a few chapters later we find out it's actually the hired hoof Zuri in disguise, when she knocks him out and steals the four fragments of the Staff. After a heated fight in the streets of Rio de Janeighro, Bravado manages to get the fragments back, but realizes they're not safe with him anymore. Obviously, the best place to keep them safe is with the one pony he knows he can always trust: Professor Daring Do.
Bravado arrives in Canterlot on a Friday afternoon, just as Daring is finishing her last class of the week. The two chat for a while, exchanging tales of heroism and treasure hunting, and Bravado asks if Daring would like to join him on his next fragment run. She responds with some sarcasm, and then gets serious and suggests splitting up. She's heard rumors of a fragment in Trottingham and volunteers to lead a separate team, since she's got a trip scheduled for there anyway.
The story follows Daring and Storm Talon's trip to Trottingham, defeating the twisted baker Sweeney Trot, and procuring the fragment they'd come looking for...but at a terrible cost. A few chapters dwell on how they're affected by the loss of Lightning Kicker, but we eventually get back to the action. Bravado has found the sixth fragment, and Ahuizotl sends taunting word that he has the seventh. Bravado and Daring board the Explorer and make for the monster's hideout back in the southern provinces. This all leads up to a climactic battle aboard the roof (including a five-minute Enemy Mine
with Ahuizotl when Zuri rears her ugly face again), as detailed on the CMoA page.
When all is said and done, the completed Staff is turned in to the Canterlot Museum, And The Adventure Continues
All in all, I cannot recommend this volume enough. If you have to pick a book to recommend if you're looking to get somepony hooked on this series and can't find Quest for the Sapphire Statue
is the one.