Well, since we don't live in "movie world", nothing would happen?

We're already used to the US' dumbass leadership making 51st state threats, and since we all know TACO, it would just be more "idiot cries wolf" noise.

There are 4.2M people in Alberta and the separatist movement has struggled to get 300k signatures.

How do you think the actual referendum would go?

Plus a referendum is not a legally binding thing that would unconditionally force separation.

Even if more than 50% voted yes, it would simply require the government to formally explore the possibility of separating. At which point they would conclude, like others already have, that it's not legally possible to separate without changing existing laws (which could also happen but would be a much longer and more complicated road).

If you want a good primer on why it's basically impossible, this video might help:

https://www.youtube.com/watch?v=0OT2IQSoVEs

It wasn't legal for the US to separate from England either. Just hypothetically, say Alberta ultimately decides to separate and proclaims their independence. They stop paying into the federal system and set up their own federal government. Ottawa says they will intervene with military force, but the US protects the separatists.

This is not a likely scenario, but it's not at all impossible either.