After several consultations between the Cenci family, the abbe and the sbirri, the following plan of action was decided upon.

The period when Francesco Cenci was accustomed to go to Rocco Petrella was approaching:  it was arranged that Olympio, conversant with the district and its inhabitants, should collect a party of a dozen Neapolitan bandits, and conceal them in a forest through which the travellers would have to pass.  Upon a given signal, the whole family were to be seized and carried off.  A heavy ransom was to be demanded, and the sons were to be sent back to Rome to raise the sum; but, under pretext of inability to do so, they were to allow the time fixed by the bandits to lapse, when Francesco was to be put to death.  Thus all suspicions of a plot would be avoided, and the real assassins would escape justice.

This well-devised scheme was nevertheless unsuccessful.  When Francesco left Rome, the scout sent in advance by the conspirators could not find the bandits; the latter, not being warned beforehand, failed to come down before the passage of the travellers, who arrived safe and sound at Rocco Petrella.  The bandits, after having patrolled the road in vain, came to the conclusion that their prey had escaped, and, unwilling to stay any longer in a place where they had already spent a week, went off in quest of better luck elsewhere.

Francesco had in the meantime settled down in the fortress, and, to be more free to tyrannise over Lucrezia and Beatrice, sent back to Rome Giacomo and his two other sons.  He then recommenced his infamous attempts upon Beatrice, and with such persistence, that she resolved herself to accomplish the deed which at first she desired to entrust to other hands.

Olympio and Marzio, who had nothing to fear from justice, remained lurking about the castle; one day Beatrice saw them from a window, and made signs that she had something to communicate to them.  The same night Olympio, who having been castellan knew all the approaches to the fortress, made his way there with his companion.  Beatrice awaited them at a window which looked on to a secluded courtyard; she gave them letters which she had written to her brother and to Monsignor Guerra.  The former was to approve, as he had done before, the murder of their father; for she would do nothing without his sanction.  As for Monsignor Guerra, he was to pay Olympio a thousand piastres, half the stipulated sum; Marzio acting out of pure love for Beatrice, whom he worshipped as a Madonna; which observing, the girl gave him a handsome scarlet mantle, trimmed with gold lace, telling him to wear it for love of her.  As for the remaining moiety, it was to be paid when the death of the old man had placed his wife and daughter in possession of his fortune.

The two sbirri departed, and the imprisoned conspirators anxiously awaited their return.  On the day fixed, they were seen again.  Monsignor Guerra had paid the thousand piastres, and Giacomo had given his consent.  Nothing now stood in the way of the execution of this terrible deed, which was fixed for the 8th of September, the day of the Nativity of the Virgin; but Signora Lucrezia, a very devout person, having noticed this circumstance, would not be a party to the committal of a double sin; the matter was therefore deferred till the next day, the 9th.