Sister Assumpta is a crazed, sadistic nun who belongs to the Mattie Hislop group.
She is first seen with a group of nuns at the parochial house on Craggy Island and they appear very taken with Ted, akin to a rabid fanbase. As he signs photos taken of him saying Mass he says he might have to cut that evening's Mass short, not divulging that he has been invited to novelist Polly Clarke's house. Unfortunately for him, Sister Assumpta, the other nuns and Dougal have also been invited and she informs him how disappointed everyone was with the Mass.
The next time she appears, Ted, Dougal and Jack have problems keeping their Lenten vows and enlist the help of the Mattie Hislop organization established to help those unable to let go of their vices. Ted is surprised when she shows up but Dougal doesn't recognize her until they mention he was wearing a blue sweater when they last saw each other. However, the priests soon regret asking for help when she subjects them to a torturous regime consisting of bathing in icy water, being dragged through the mud tethered to a tractor, Chinese burns (which have no effect on Dougal), being shot at and having their mattresses replaced with bricks.
After they discover that their enemies on Rugged Island have broken their Lenten vows, Ted and Dougal return home to discover that Sister Assumpta has eaten all the chocolate eggs that John and Mary gave as a present, breaking her own Lenten vows. She informs them that she can't go home to her fellow nuns until after Easter and asks what she can do to repay them. Ted decides to send her to Rugged Island to inflict a similar punishment on their rivals.