By working upon his fears, his suspicions, by stories of plottings against his life and his kingdom, she was to infuriate him; and then, while his rage was at its height, the opportunity for action must be at hand.
The marriage of Charles's sister Margaret with the young Protestant leader Henry of Navarre, with its promise of future protection to the Huguenots, was part of the plot.

It would lure all the leaders of the cause to Paris.

Coligny, Conde, all the heads of the party, were urgently invited to attend the marriage feast which was to inaugurate an era of peace.
Admiral Coligny was requested by Catharine, simply as a measure of protection to the Protestants, to have an additional regiment of guards in Paris, to act in case of any unforeseen violence.
Two days after the marriage, and while the festivities were at their height, an attempt upon the life of the old admiral awoke suspicion and alarm.

But Catharine and her son went immediately in person to see the wounded old man, and to express their grief and horror at the event.
They commanded that a careful list of the names and abode of every Protestant in Paris be made, in order, as they said, "to take them under their own immediate protection." "My dear father," said the king, "the hurt is yours, the grief is mine." At that moment the knives were already sharpened, every man instructed in his part in the hideous drama, and the signal for its commencement determined upon.

Charles did not know it, but his mother did.