"Venus and Cupid in the Forge of Vulcan" of Rubens is a masterpiece of the Royal Museums of Brussels. This painting has completely changed in size, shape and meaning since its creation. The painter first enlarged it to paint the whole right side with Venus and Cupid. But the left part has been replaced by a fake! It is therefore a half Rubens... Why did Rubens enlarge his first painting? What had he painted initially? The investigation will lead us to Brussels and Antwerp in Belgium, to the Het Loo Palace in the Netherlands and to Dresden in Germany.