Ah, the Performance Improvement Plan. In upper management, we call this "managing people out"
it will usually start out of no where. you start getting some feedback about your performance. it will escalate. you will start having weekly meetings with your boss. your mid-year review will summarize your "poor" performance, the feedback will continue. you will get no feedback on how good you are doing. it usually culminates in a poor end year review followed by a PIP OR they will put you on the PIP before your year end review. At end of PIP you will get put on 30 day final warning. By this time, its over but has been the strategy from the start of the process.
some collected advice I have received over from someone in legal/employee relations, someone who is in investigations, and from a couple of external employment attorneys. Read all this
Write everything down. Everything. Take detailed notes (dates, times, who was present, summary of conversation) of conversations and meetings with your manager or other meetings where your manager is present. Save emails and chats where you are having conversations and include those as attachments to your "notes" email (below). My advice, record your conversations, use the recordings to take your detailed notes, then save or delete the recording (consult your state wiretapping laws). It's a little work but it pays off.
To make your notes "official" send an email to yourself (work email) with these notes (include relevant attachments) for each meeting or interaction. Then, print this off, physically. Do not email it to your personal email account. If they are looking to get rid of you this is one way they can fire you for cause. If you cannot print physically, then take a photo of your screen that has the email notes opened up with your phone. Still send the email to yourself (work email) because it now becomes discoverable if there is any litigation.
Key is to show you are performing and that your manager is contradicting themselves, or providing you with poor management, poor direction, vague instructions, etc. You have to be able to paper these people to the point that they give you a severance to avoid legal issues.
Take notes on feedback you receive from your manager, the frequency of your interactions with them and if they ever provide you feedback or guidance. I know some people here who never speak to their managers for weeks. Note this as well. Use snippet to take a screen shot of your calendar meetings showing how often your boss is reaching out to meet with you for one on ones. include this in your notes email (from above).
Your notes will pay off. trust me. if you keep this up for a couple of months or longer there is no way in hell your manager is going to be able to recollect or refute things you have documented.
DO NOT contact HR. DO NOT contact Wells Legal. These groups are here to protect the firm and your manager. DO NOT EVER write a response to your manager and CC Legal or HR. If you contact HR or Legal trust me, they will contact your manager even though they say your conversations are confidential.
If you have been here a long time and/or are older, consult with an employment attorney to build a strategy and plan. Its worth the thousand or so bucks you will pay.