Assuming that the agreement is voted down, the margin of defeat is important.
If you lose about 30, 40 or maybe even 50 votes, it is expected that you could stand up in Parliament and say that you will come back to Brussels and try to change the agreement
If you agree to read concessions with the 39, EU, this could be enough to convince some British parliamentarians to line up with a second parliamentary vote in May.
One caveat to this scenario is that the rest of the EU has shown little appetite to reopen the talks. The President of the European Commission Jean-Claude Juncker warned the United Kingdom that the agreement on the Brexit of May "is the only agreement possible"
If May were to lose heavily, chaos is once again destined to go down on British politics.
The range of results runs from a challenge to leadership, a resignation of May, a general election in the UK, a second referendum on membership of the EU, a temporary stop on the withdrawal of the Grand Brittany, or a kind of combination of all of the above.