There is a complicated back story concerning Rachel's involvement with Mitch, but here are the main details:

In 1980, Rachel found herself desperate for a certain bit of information and Mitch was the only one with the knowledge. In order to gain Mitch's confidence, Rachel was forced to sleep with him.

Nine months later, Rachel gave birth to their son, Matthew Cory. Rachel won custody and intended to raise him with her husband Mac Cory. Mitch wanted to raise his son himself, so he arranged for Matthew to be kidnapped and brought to him. Mitch's plan was undone and he was sent to prison.

Mitch returned to Bay City in 1986, where he stayed for the next four years. Things were rocky between him and the Cory family, but eventually they all became friendly. Mitch was able to provide invaluable support to Rachel when Mac died in 1989.