The children are invited by Mr. Benedict to reunite on a scavenger hunt across Europe. When they arrive, however, they're met with unsettling news: Mr. Benedict and Number Two have gone missing. Despite being told it's too dangerous, the kids want to help find their lost comrades and decide to sneak away and start the search on their own. Meanwhile, Mr. Benedict and Number Two are unharmed, but they have been kidnapped by Dr. Curtain. He is now deep into a suspicious new endeavor - happiness.

Stowaways aboard a majestic ship, the kids must try to blend in and find the next clue. Hot on their tail, Rhonda, Milligan, and Ms. Perumal board a dirigible to catch up to them. Mr. Benedict and Number Two cannot hide their distrust from the eerily calm Curtain after observing the unnervingly joyful behavior of his devotees and discovering that he is using neurological techniques to create artificial happiness. Back at sea, the water polo team sneaks suspiciously towards the kids' stateroom...

After narrowly avoiding capture by the imposturous water polo team, the kids try to stay hidden, but are distracted by an underground casino. At Curtain's compound, an acolyte is found completely unresponsive, suggesting a frightening symptom of his methodology, while Number Two works on an intricate escape plan. After negotiating with the Captain, the kids arrive successfully in Lisbon, but are immediately on the run again when Curtain's Head of Security spots them at the port of entry.

Heading to the train station, the kids realize that Curtain must have sent multiple teams after them. Benedict, in his state of involuntary elation, praises Curtain for his discovery.

The kids call in the help of an old friend as they scour the countryside for clues to Constance's whereabouts. Meanwhile, Constance finds herself imprisoned by a surprising foe who hopes to shed light on an old failure.

After learning the location of Dr. Curtain's compound, the Society prepares to strike but is beset by Greys and must split up to escape. Meanwhile, Mr. Benedict tries to bond with his twin in an effort to gain intel, but is sidelined as Dr. Curtain sows discord between Mr. Benedict and Number Two. While on a detour, Constance finds herself surprisingly intrigued by Curtain's TV special. After discovering the scope of catatonia among his acolytes, Curtain is horrified but denies responsibility.

Mr. Benedict and Number Two's plan to subdue Dr. Curtain backfires. After arriving at the compound, The Society regroups to rescue their captured comrades. Meanwhile, Dr. Curtain hosts a celebration to announce his plans for global expansion. After Sticky cracks the method behind Dr. Curtain's happiness technique, the kids struggle to devise a plan to reverse the effects before all is lost. The Society then attempts a daring escape and find themselves fleeing with a surprising companion in tow.

Banding together to stave off disaster at a haven from Number Two's past.