The sponsor of the project has completed the Project Charter. The Charter describes the business need of the Project, a high level description of the Scope, and may tie to the organizations Strategic Plan.
These are all tied to the Business Case that was made in order to get the project funded and prioritized.
It is the official document that formally authorized the project, gives the Project Manager the authority to get the project started, and may contain items such as the high level requirements, assumptions and constraints, hard dates, and a high level budget.
It also represents the sponsors view of the project. This is important because it will ultimately be compared to the Scope Statement that will represent the Project Managers view of the project - and they should be compatible!
More on that later.
You are a good Project Manager, so you review all of the abovementioned documents and....
You think the project is doomed to failure.
There are both practice and ethical considerations here! Practically, do you want to be associated with a project that is going to fail? Ethically, you are supposed to be transparent and immediately disclose any pertinent information about your project.
Doesn't the fact that the project is will fail qualify!
So, you have a responsibility to "reject or accept" the project. The project that, in your professional opinion, is impossible.
So you should "reject' it!
But you really like making car payments, eating, having a place to sleep....
But you can't really "accept" the assignment either - if the project is doomed, you need to report that!
As an experienced Project Manager, you realize there is a different path! You see an opportunity to create circumstances that will benefit the organization by setting up the project for success!
So you go to work and analyze the project to understand what it will take to make the project successful! Is it more important to be on time or on budget? Or does the schedule trump all?
Can you speed things up with contract support? With pre-assignment of key resources? With access to key stakeholders? All of these things and more are considered.
So you formulate a plan, and you meet with the sponsor. You use this as an opportunity to educate them on the Project Planning process. You explain the challenges and describe the circumstances under which the project will be successful!
And they hear you and immediately give you what you need!
Well, sometimes. Sometimes there is compromise. And sometimes you are told to quit complaining and just do the project!
Which is OK! You met your ethical requirement to inform and be transparent. You provided a path that would have led to success, and your plan was rejected.
So you do your best to deliver! We all have bosses!