In the novel They Came to Baghdad, Sister Thérèse is a nun at a convent in Baghdad.
Late in the story, it is revealed that she is an operative of of "The New Order" secret organisation. Edward Goring brings Victoria Jones (who has offered to help the "New Order") to the convent. Sister Thérèse disguises Victoria as a nun and then escorts her to Damascus for her to intercept and impersonate Anna Scheele who is transiting there on her way to Baghdad. Although "The New Order" accepts Victoria's services, they obviously take no chances as she notices that Sister Thérèse is armed with an automatic pistol and doesn't let her out of her sight for a moment.