In 1655, he re-appeared as a member of a cavalry regiment in Scotland and took part in a plot to take control of the local army.
Sindercombe joined his plot to assassinate Cromwell in hope of restoring the Puritan republic as they saw it.
First, Sindercombe rented a house in King Street in Westminster, where they intended to shoot Cromwell when he rode past in his coach.
Sindercombe's group then intended to shoot Cromwell when he left for Hampton Court, as he customarily did every Friday.
They broke the hinges of the park gates to facilitate their escape, and John Cecil began to follow Cromwell and his entourage.
Boyes made an explosive device out of gunpowder, tar and pitch, and the group planted it in the palace chapel on 8 January 1657.
With Toope's aid, Thurloe learned Sexby's part in the plot and presented his findings to the Parliament.
On 9 February 1657, he was found guilty of High treason when both Cecil and Toope testified against him and was sentenced to be hanged, drawn and quartered.