The plot of the ballad "Sister Helen" can be summarized as follows: Sister Helen is in a dialogue with Little Brother. Through this conversation, we learn that, in medieval Ireland, Sister Helen is killing her beloved, Keith of Ewern, who betrayed her by marrying someone else. She begins her revenge plot on the morning of his wedding with another woman by slowly—over three days—melting a wax figure she made of him.

The characters in the ballad include the angry and persistent Sister Helen, Little Brother, and Keith of Westholm, who comes to plead with Sister Helen on the dying Keith of Ewern's behalf. Westholm tells her Keith of Ewern wants to meet with her, but as far as Sister Helen is concerned, it's too late for that. Then Keith of Keith, Keith of Ewern's father, arrives to make a plea for his son's life, kneeling in the road. Keith of Ewern's new wife also shows up, golden haired and sobbing, hoping to save her husband. None of these characters can persuade Sister Helen to alter her decision.

The main supernatural element of the poem is Sister Helen's witchcraft. Melting the effigy does succeed in sickening and killing the real Keith of Ewern, so the witchcraft works.