Light is called in by the task force in order to help them on the Kira case.  They don't originally tell him they think there a two Kiras but he deduces this on his own (obviously).  The task force then sends a fake message from Kira to the copy-cat in hopes of a response.  The second Kira responds and shocks both Light and L by mentioning the "eyes" and Shinigami.  Light becomes determine to stop the second Kira before they can do more harm.  Meanwhile, the second Kira, named Misa Amane, learns from her Shinigami what it takes to kill one of their kind.