1) You need talk with HR to see what is the company policy for performance issue, big company normally requires manager warning first, then HR interview, then performance improvement plan. If the policy were not followed, you will get some points
2) you might have to sign the plan, but you should write up your arguments with the facts, email to you manager and CC to HR, trying a meeting with your manager for better communications.
3) Find someone in company you could talk and get advice.
3) Try to collect any evidence if you felt decriminated, especially emails, comments, your annaul reviews, and try your best not to give your manager any execuses.
some guys in TX does descriminate Chinese because they thought Chinese are silent nerd. I have two friends filed desrimination law sue after being fired and they both won the cases in couple years for some money compensations
Only two cents, Good luck!