It's an ivory sculpture of Adam and Eve, carved in 1627 by Georg Petel, a friend of Reubens's, who, according to Breitwieser, gifted him the piece for his 50th birthday. The carving is a masterpiece, just ten inches tall but dazzlingly detailed, the first humans gazing at each other as they move to embrace, Eve's hair scrolling down her back, the serpent coiled around the tree trunk behind them, and the unbitten apple, cheekily, in Adam's hand, indicating his complicity in the fall of man, contrary to the book of Genesis. “It's the most beautiful object I have ever seen,” says Breitwieser.

Georg Petel’s ivory sculpture of Adam and Eve, stolen from—and later returned to—the museum at the home of Peter Paul Rubens in Antwerp. Picasa

The ivory sculpture is sealed beneath a plexiglass dome fastened to a thick base, resting on an antique dresser. Breitwieser's first objective is to remove the two screws that connect the dome and the base. There's no camera here, and only one guard is in motion, poking her head in every few minutes.

The tourists, as usual, are the problem—too many of them, lingering. The room is filled with items Rubens had amassed during his lifetime, including marble busts of Roman philosophers, a terra-cotta sculpture of Hercules, and a scattering of 17th-century oil paintings.

Patience is needed, but a moment soon comes when it's just Kleinklaus and Breitwieser alone, and in an instant he unfolds the screwdriver from the Swiss Army knife and sets upon the plexiglass dome. Breitwieser is shorter than average and tousle-haired, with piercing blue eyes that, for all his stealth, are often animate with expression. He is lithe and coordinated, and uses athleticism and theater in his work. Maybe five seconds pass before Kleinklaus coughs and he vaults away from the carving, reverting to casual-art-gazing mode.

It's a start. He has turned the first screw twice around. Each job is different; improvisation is crucial—rigid plans do not work during daytime thefts, when there are variables too numerous to preordain. During his previous trip to the museum, he had studied how the Adam and Eve was protected and had also spotted a convenient door, reserved for guards, that opened into the central courtyard and did not appear to have an alarm.

Over the course of ten minutes, progressing fitfully, Breitwieser removes the first screw and pockets it. He does not wear gloves, trading fingerprints for dexterity. The second screw takes equally as long.

Now he's set. The security guard has already appeared three times, and at each check-in Breitwieser and Kleinklaus had stationed themselves in different spots. Still, the time elapsed in this room has reached his acceptable limit. There's a group of visitors present, all using audio guides and studying a painting, and Breitwieser judges them appropriately distracted.

He nods to his girlfriend, who slips out of the room, then lifts the plexiglass dome and sets it carefully aside. He grasps the ivory and pushes it into the waistband of his pants, at the small of his back, adjusting his roomy jacket so the carving is covered. There's a bit of a lump, but you'd have to be exceptionally observant to notice.

Then he strides off, moving with calculation but no obvious haste. He knows that the theft will swiftly be spotted. He'd left the plexiglass bell to the side—no need to waste precious seconds replacing it—and the guard will surely initiate an emergency response. Though not, he's betting, quickly enough.

From the room with the ivory, the museum layout encourages visitors to ascend to the second floor, but Breitwieser pushes through the door he'd seen on his earlier trip, crosses the courtyard toward the main entrance, and walks past the front desk onto the streets of Antwerp. Kleinklaus rejoins him before they reach the car, a little Opel Tigra, and Breitwieser sets the ivory in the trunk and they drive slowly away, pausing at traffic lights on the route out of town.

Stéphane Breitwieser robbed nearly 200 museums to amass his secret art collection. CHRISTOPHE KARABA/EPA/REX/Shutterstock

Crossing international borders is stressful but low-risk. They travel from Belgium to Luxembourg to Germany to their home in France without incident, just another young, stylish couple out for a jaunt. It's the first weekend of February 1997, and both are only 25 years old, though Breitwieser's already been stealing art for a while.