Charlie's dad has been told that he has Alzheimer's disease, a kind of dementia. Dad has to explain what this means to help prepare Charlie for what might happen. To help themselves deal with this, the family think of the Alzheimer's as an unwanted guest in their home, and they call him Al, short for Alzheimer's. Dad's dementia means that he might say or do strange things, but thinking of Al as a person means the family can blame him rather than Dad...