As they walked along the Calle de las Cadenas de San Gregorio to the Plaza Mayor, they were only two tourists in a large, open space with hundreds of people. When they were about halfway across the plaza, they heard a new sound—the deep, throaty sound of a motorcycle engine. The engine grew louder as the motorcycle came around a corner somewhere behind them. Remi started to turn to look over her shoulder, but Sam put his arm around her and whispered, “Don’t look or you’ll scare them off.”

The motorcycle sped up directly behind them, and Sam turned suddenly. On the cycle were a driver and a man riding behind him. Both men were wearing helmets with tinted visors over their faces. As the motorcycle swooped in, the driver made an attempt to snatch Sam’s briefcase out of his hand, but Sam held on to it, tugging back, as the driver pulled. The power of the cycle added to the driver’s force, but Sam trotted alongside, still holding on. When the second man saw the way Sam held on, he joined the struggle, grasped the briefcase with both hands, and wrenched it away. The driver gunned the engine and accelerated, and the motorcycle roared off around the side of the Plaza, turned, and disappeared up a narrow street between tall buildings.

Sam held up his empty hand so Remi could see it.

“Sam! He stole your new briefcase!”

He smiled. “A little engineering project.”

“What are you talking about? Those men stole your briefcase! We’ve got to call the police!”

“No need,” he said. “Those are the two we noticed about a week ago outside San Gregorio. I saw them watching us a few times since then. They were too interested to be nobody. So I bought the briefcase and began my project.”

“Your briefcase is an engineering project?”

“Didn’t I say that?”

“Stop being mysterious and tell me what you’ve done.”

“You know those booby-trapped bags of money that banks give to bank robbers?”

“The ones that blow up and cover the thief with indelible ink? Oh, no. How did you even get an explosive on the plane?”

“I didn’t use explosives. This one works with springs. Undo the latch and the first spring pops the case open wide, and that allows the second to spring upward and push a piston, like a jack-in-the-box. The cylinder is full of ink. I bought the briefcase, the springs, and the ink here.”

“What would have happened if the librarian had inspected it?”

“He didn’t open anything for the first two days, so why do it later?”

“What would have happened to him?”

“He would have a bright blue face. ‘Azul,’ as they say here.”

“You couldn’t just watch these men and not play some dumb prank?”