Britain could have a new Prime Minister just 49 days before Brexit .
Labour swung behind Boris Johnson's bid for a general election in December - the first one since 1923.
Whoever wins will steer Britain's policy on leaving the EU for years to come, and their first days in office will be crucial deciding what happens next.
Boris Johnson and Labour have radically different approaches. So what will happen to Brexit if either of them get into power?
Here's a basic explanation.
Is Boris Johnson's Brexit deal dead?
Nope. Well not exactly.
The deal is still active and agreed between the UK, the EU and 27 European leaders - so it can be taken up by whoever gets into 10 Downing Street, Labour or Tory.
But the Bill that puts it into law will be parked until after an election, when it'd have to be brought back to the UK Parliament from scratch.
It'll be easy to bring the Withdrawal Agreement Bill back and pass it if Boris Johnson wins a majority.
If he doesn't get a majority it's a different story. So the PM is gambling it all on winning the house.
What happens if the Tories get a majority
Boris Johnson would have achieved his goal of clearing opposition to his Brexit plan.
He could bring back his Withdrawal Agreement Bill and, this time, get it through the House of Commons without it being whittled down by angry MPs.
No doubt he would still face opposition, but if he had an overall majority - of MPs buoyed by an election victory - he'd likely be able to get it through the Commons before the January 31 deadline.
There could still be a hold-up when the Bill got to the Lords, and the European Parliament, which both need to ratify it. But the path would be easier.
If he still faced opposition, Mr Johnson has not ruled out leaving with no deal at the end of January.
What happens if Labour get a majority
Labour has vowed to try to negotiate its own, softer Brexit deal - one that protects workers' rights, has alignment with the single market and puts the UK in a customs union with Brussels.
You could expect to see Jeremy Corbyn going to Brussels in a bid to reopen talks with EU chiefs.
Brussels leaders have previously been reluctant to do this, but a change of leadership in the UK would be a clear incentive to reopen negotiations.
Whatever Labour come up with, that would then be put back to the people in a "confirmatory" second referendum.
It's likely people would be asked to choose between Remain and the Labour deal. Labour would only decide how to campaign in that referendum at some point in 2020 at a special conference.
All this would almost certainly require another delay to Brexit.
What happens if the Lib Dems get a majority
This would be unprecedented but... they've said they'd revoke Article 50 . The Lib Dems also back a second referendum.
What happens there's a hung parliament
We know you hate to hear it, but... probably more of the same.
How Brexit unfolds will be defined by what groupings can be formed between Remainers - including small parties like the Lib Dems, who are widely expected to make gains - and Brexiteers.
Expect more running battles, attempts by backbenchers to delay Brexit, or a surprising majority around one thing or the other.
A hung Parliament could boost calls for a second referendum, because some would see it as the only way out now an election has been exhausted.
But whether enough MPs would back it for it to actually happen, through a vote in Parliament, is a totally different matter.
