Pretty much this. And if they don't follow through/follow up on this, the problem will get much bigger.
Another thought- it's possible that your supervisor's manager is aware of the action being taken against you. You might be able to get better advice talking to someone in a different reporting chain if you can find them.
Whether that would actually help here and whether such markings have any weight in whatever legal system the OP is operating within are different questions, of course.
Either way things can't go back to the way they were. You're either getting fired, or you're going to choose to leave. This means changes are coming. So you've got to take charge of how your life is going to change.