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"It's a long flight," one of my fellow lobby journalists asked, "will you spend any time working up a Plan B in case the vote falls?"

"Nice try," the prime minister answered.

And however many ways the question was put to Theresa May during a 20 minute press huddle on board the government's Voyager plane, she was resolutely and determinedly not going to let much slip.

What will she do if she can't get her vote through? Is there a plan B? Could there be a Norway-style relationship with the EU? Could there be another referendum?

Westminster's awash with speculation and gossip about what might happen next if her Brexit compromise is killed off by her colleagues.

It's not surprising.

Unless she pulls off a political miracle her plan is on track to, at least at the first time of asking, be rejected by the Commons.

That would be the central policy of a minority government being chucked out - an event that even in this strange times, might be the end of her leadership and the end of the government.

Of course the defeat might not happen.

If it does, the scale of it will be hugely important. If it's only by a few votes, perhaps she would try again. If it was by a thumping 100, I'm not sure she would be able to.

That raises the prospect of another last-ditch effort to crank something else out of Brussels, even a new prime minister, or a general election.

You can forgive Westminster then perhaps for indulging in rather a lot of speculation - it's not idle gossip, but the hunt for clues about what's next in these vital days.

But all Mrs May would really give on the way to Buenos Aires was a slightly more emphatic plea to backbench MPs to think about their constituents, rather than themselves - an obvious, if veiled criticism of her Brexiteer backbenchers who think her plan leaves the UK too closely tied to the EU after Brexit.

Goodness knows what she really thinks of them in private.

Some of her allies have had pretty colourful words for them of late, many of which are too rude to write down in a respectable blog.

But you can imagine the drift, the frustration, at the prime minister's proposal not being seen as "pure enough" as one senior politician described it, to pass.

"They'd have bitten our hands off in 2016 for this," they said. "Now it seems nothing will ever be good enough."

But instead the prime minister gave her familiar lines on why she could not support a customs union-style deal or another referendum, her already familiar mantras about why the deal she has negotiated is the right one.

Talk to her confidants in Number 10 and they say her fears about chaos and disruption if the vote fails are genuine, insisting there is no theorising or strategising about what would happen next.

One insider suggested at that moment "Parliament would have their hands on the executive", downplaying the idea of Number 10's ability to argue for a second vote.

But if nothing comes along to change the dynamics, before too long discussions about what happens next may well be ones that the prime minister simply has to have.

If she wants to avoid them she may need to find a new script, and fast.