Diary of a Drug Fiend encapsulates much of Crowley's core philosophy concerning Thelema and his conception of True Will.

The story follows Sir Peter Pendragon a noble, aristocratic World War I veteran pilot who has come into a large inheritance following the death of his paternal uncle.

Prior to the war Pendragon had been a medical student, and now finds himself dealing with depression and lacking direction.

While in Italy, their luggage and valuables are stolen by a school acquaintance of Pendragon's named Elgin Feccles, whom they had hired as a guide.

Back in England, the protagonists find themselves desperate after their drug supply diminishes due to laws that had been passed during their honeymoon.

Their decline in health and financial problems leads to the couple making several unsuccessful attempts to quit heroin.

Peter's initial interest in financing the purchase of the cocaine factory upsets Louise so much that she threatens him with divorce and to leave him for Lamus.

Lamus then frees Peter and Louise from their addictions through the use of Magical techniques, aimed at mastering True Will and releasing the individual from sloth, self-destructive impulses and craving.