Sister Assumpta is a crazed, fanatical nun who belongs to the Mattie Hislop group, whose job it is to help people who are suffering from addictions.
She is first seen with a group of nuns at the Parochial House on Craggy Island and they appear very taken with Ted Crilly, 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. He rushes through that evening's Mass but 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 McGuire and Jack Hackett have problems keeping their Lenten vows, and Ted enlists 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, upon learning that a nun is going to stay over, a sober Jack frantically runs away from the Parochial House for the whole session. Ted soon regrets asking for help when she subjects him and Dougal to a torturous regime consisting of bathing in icy water (which is only the prelude), 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.