Are the sins of the past threatening the present . . . ?

1686, The Netherlands

After getting Master Mercurius jailed and nearly put to death with one of his schemes, the Stadhouder, William of Orange, has finally left Mercurius in peace.

But Mercurius is not able to remain in Leiden for long. A friendly debate on the sin of witchcraft has been proposed between the University of Leiden and the University of Leuven. And the scholars are to meet part-way in the city of Maastricht.

When researching the local witch trials from seventy years ago, Mercurius comes across astonishing charges that could not possibly be true. But the opposing side brings forth a witch-finder as a witness who is adamant that the women he charges bear the signs of the devil.

Before the debate can continue, the witch-finder is found brutally murdered. His body was left inside the library, locked from the inside with no other available exit.

Could this be the work of Satan? His wrath provoked by the investigation into local witch trials? Or is the culprit someone grounded much more in reality . . . ?