The Opposite of Murder

What if the only way to prevent a murder is by confessing to it?

Jemma Stelling has confessed to a crime - one she can't have committed. She has an unshakeable alibi: when Marianne Upton was stabbed to death outside her home, Jemma was at the police station confessing to a deadly obsession with Marianne and a plan to kill her in precisely the way she was killed.

So is Jemma innocent? Or is she an ingenious, cold-blooded murderer?