The Good Sister follows a couple who is working to put their crumbling marriage back together, when the wife receives a letter informing her that she has a long-lost twin sister. First thinking that the new family member will bring the couple closer together, the husband then begins an affair with the sister and discovers that she is treacherous and dangerous.

When marriages fall apart, divorce can turn ugly - even deadly. From blushing brides turned cold-blooded killers to smitten grooms with shocking dark sides, the criminal psychology behind deadly divorces is examined. Guiding viewers through actual stories of love gone wrong are psychotherapist Stacy Kaiser and forensic psychologist Dr. Brian Russell, who analyze each couple to better understand how the marriage turned from flawed to fatal. .

Three paranormal roommates, a ghost, a vampire, and a werewolf, struggle to keep their dark secrets from the world, while helping each other navigate the complexities of living double lives.