A plan can look impossible because nobody has listed what is actually available yet. The deadline is real. The assumptions around it still need checking.
I've built and exited a holding with 26 startups. That history doesn't tell me whether the next plan can work. I still have to examine the work in front of me, including the parts of the plan that sound settled because somebody put them in a presentation.
I'd start with the required outcome. What has to be true at the end, and which parts are preferences? Then look at the people who are actually available, the dependencies and the work already promised. A name on an org chart isn't the same as time available this week.
The inventory should include constraints that won't move. A regulatory requirement needs a different conversation from a reporting format someone happens to like. Treating both as equally fixed can make a workable project look impossible. Treating both as negotiable can make it dangerous.
Once that picture exists, the trade becomes clearer. Reduce the scope, change the sequence or move the date. If none of those options produces an acceptable result, say so with the evidence attached. Optimism doesn't create another engineer, and a red status slide doesn't explain which engineer you need.
The point of the inventory is to find a decision somebody can make. Sometimes it supports the plan. Sometimes it gives you a solid reason to stop.
Before I accept “we can't,” I want to see what the conclusion is built on. Before I promise “we can,” I need the same thing.