The Mysterious Benedict Society is reunited when Mr. Benedict invites the children to his mansion with the expectation of sending them on a globe-trotting scavenger hunt. But when Mr. Benedict and Number Two go missing while in Europe, the kids defy the adults' orders and sneak off to find them on their own. Mr. Benedict and Number Two have been kidnapped by Dr. Curtain, who has bounced back after the failure of The Emergency by claiming credit for its destruction and capitalizing on that fame to push his new Happiness Revolution – which is really a subtle form of manipulation creating an artificial feeling of euphoria. Dr. Curtain and his Happiness acolytes live on a compound in rural Italy and are making plans to spread their joy to the world. The kids cross to Europe by stowing away on a ship and use the clues left for them by Mr. Benedict to get closer to finding out the truth of their whereabouts. Meanwhile, the children are pursued by both the adults of the Society (Rhonda, Milligan, and Ms. Perumal) who are concerned for their safety, as well as a Water Polo Team with nefarious intentions. The intrepid members of The Mysterious Benedict Society scour Western Europe as they piece together the mystery in front of them, dodging unexpected dangers and new and old adversaries at every turn. When everyone reunites at Dr. Curtain's compound, they realize that Curtain's methods lead to unexpected side effects. The Society enacts a swift escape and retreats to a safe location to prepare for a final confrontation, but they must find a way to undo the damage of Curtain's methods on some of their own before it's too late.