Lethal Vows is based on the story of Dr. Richard K Overton, who gained attention after his first wife, Dorothy Boyer, who raised four children with him, told authorities that Overton “tried to murder [her] through a process of slow poisoning” after they divorced in 1969. The couple had split after Dorothy learned that while they were married, Overton had secretly married another woman, Caroline Draper, with whom he had a child. Then the doctor married a third time to a woman named Janet. Nineteen years later she would refuse to divorce Dr. Overton and pay for it with her life.
There is an out of print book about this case: Final Affair: The Shocking True Story of Marriage and Murder. I can’t speak for its worthiness as I have not read it to date, however other Amazon reviewers give it an average four star rating.
Now, on to the movie -