If a plenary indulgence be applied to a certain soul in purgatory there would be no more need to pray for that soul.

The Catholic Answers

A plenary indulgence, of course, would be able to liberate a soul from purgatory. But we cannot know that we have satisfactorily fulfilled all the conditions necessary for the gaining of a plenary indulgence, and we cannot know for certain that God has actually accepted it, it gained, on behalf of the particular soul we have in mind. We know the general principle that indulgences are beneficial to the souls in purgatory, and we gain and offer them to God, leaving all questions as to their application to Him.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty