What will happen if I refuse to sign a performance improvement plan

Over a 6 month period I was extremely overloaded working 60+ hrs/ wk , on the weekend, during vacatation, and holiday. Based on my workload, I was expected to perform the job of 3 project managers. Unfortunately no change occured until after my peers started complaining about my performance. When the changes finally occured, one of the major projects I had was assigned to a new project manager who is the sister of the support manager on that project. I was very dissapointed and unmotivated which reflected in my job performance over a 3 month period. I recently went through a review cycle and was told that I would be put on a performance improvement plan. I dont' agree with the performance appraisal nor the performance improvement plan. I told my manager that I'm not comfortable signing the document because I feel as though if I sign, I'm acknowledging that I am solely responsible for not performing to unrealistic expectations over that time frame.
I really don't want to sign, but fear I could be terminated for not signing. Please let me know what are some possible legal recourses.

you may also want to ask the company to put what is happening in writing and explain to them that you may have attorneys review the situation...try speaking to a higher level of management about the issue; they may override the other managers. If none of these efforts resolves the issue, let us know.