C.S. Lewis' "The Screwtape Letters" is a series of instructional letters from a senior demon, Screwtape, to his nephew apprentice, Wormwood. The letters chronicle Wormwood's efforts to keep a man, known as "the patient," from becoming a Christian.
In the story's opening, Screwtape's letters are encouraging and offer Wormwood advice on keeping the patient away from God. Wormwood is portrayed as blundering and, as the story continues, Screwtape's letters become more hostile and sarcastic.
The "patient" is a young man in England during World War II. In the opening chapters, he is a passive churchgoer seen as vulnerable to Wormwood's efforts. As the story continues, he becomes more involved with church and stronger in his faith. The patient eventually marries a Christian woman, becoming part of a charitable, loving circle of friends and family.
As the patient grows in Christianity, Screwtape becomes more harsh and impatient toward Wormwood and his failing as a tempter. Screwtape warns Wormwood that if he does not deliver the patient to "Our Father Below," Wormwood must take his place and be devoured to meet the senior demon's hunger.
Throughout the story, Screwtape advises Wormwood on the nature of humans. Screwtape offers insights into people's thoughts and actions and tries to teach Wormwood to exploit their weaknesses. At one point, Screwtape tells Wormwood that sin does not have to be grandiose like murder or adultery; it is the collective small thoughts and actions that lead people to hell. He explains to Wormwood that humans don't recognize their sins because they are small and that Wormwood can be successful by taking advantage of their ignorance.
In the end, the patient dies a man of the church and goes to heaven. Wormwood fails to tempt him to hell and Screwtape devours his nephew.