When Marylou was a young woman and pregnant, she given a radioactive concoction to drink by her doctor, Wilson Spriggs. She was not told it was radioactive, she was told it was a vitamin supplement and she drank it. She and hundreds of other pregnant women were unknowingly dosed in a secret study. (This actually happened to real women in the USA.) Years later her daughter, with whom she pregnant when she was dosed by Spriggs, developed cancer and died. At that time Marylou still did not know about her exposure to radiation but she did find out about it much later when the secret study was revealed. Her anger at the callous young physician who dosed her with poison grew and festered.
Now an old lady in her seventies, Marylou has discovered that Dr. Spriggs is living in Tallahassee, Florida. So she moves into a house just a few blocks from where Spriggs is living with his daughter and her family. Marylou is out for blood: Dr. Spriggs must admit his guilt and accept his punishment. Dr. Spriggs must die.
But how to kill him? Marylou, going by the name Nancy Archer (the giant woman in the old movie, Attack of the 50 Ft. Woman),
can't quite figure out how. After all, she is a frail old woman. Then she hits on the idea of insinuating herself into her enemy's family, with the view to gaining access to the old man. But in the process, she comes to know Spriggs' family much better than she ever intended and nothing seems to turn out the way she envisioned, starting with the fact that Dr. Spriggs has Alzheimers and his memory is failing.
For a better review, see http://www.nytimes.com/2011/02/13/books/review/Willett-t.html?_r=0