Elisabeth Moss to Guest Star on Mercy

You don't see this very often: a series regular on a current hit show guest star on a different show later this season.

Elisabeth Moss, the critical darling of Mad Men for her portrayal of Peggy Olson, will guest star on NBC's medical drama Mercy.

She'll play Lucy Morton, a cancer patient that has lost her hair from treatment. She'll evenl don a head scarf to cover her baldness.

Sources say most of Moss' scenes will take place with Veronica and Dr. Harris. Her Mercy episode will air on November 4.