More than two and a half years since citizens of the U.K. voted to leave the EU — and following two years of torturous negotiations — the British government is still struggling to win approval for its Brexit deal from parliament.

Following a historic defeat of Theresa May’s “Plan A” in the House of Commons in early January, MPs are gathering Tuesday to debate and vote on a potential Plan B, as well as several amendments to it.

The hope is that by 9pm Tuesday Britain will finally have a cohesive plan that they can put forward to the EU. If this is considered workable by Brussels, it could then shape the direction of Brexit and the future relationship between Britain and its closest trading partners.

If it fails, however, there’s a very real chance that the U.K. Parliament will wrestle control of the Brexit process from Theresa May’s government, leading to further months of stalemate and potentially no Brexit at all.

The stakes for the EU, Britain, and May could not be greater. Here’s a rundown of the key figures and amendments to watch out for on a day that’s set to produce high political drama:

John Bercow

The speaker of the House of Commons is traditionally a neutral position in British politics, but the current speaker, John Bercow, has been criticized for playing a more activist role, especially when it comes to Brexit. Earlier this month, when Parliament first voted on Theresa May’s plan, Brexit supporters were furious that Bercow allowed an amendment to be tabled that would require May to present a Plan B within three days if her first plan failed. Critics said that was an unreasonably quick turn around that would favor parliamentary control of the debate.

Crucially, Bercow will determine which amendments MPs will debate and vote on Tuesday.

Deal or No Deal

Much of the hand-wringing in Parliament concerns the possibility that the U.K. could crash out of the EU with no deal—as in no plan—for the future. MPs who favor remaining in the EU—as well as some who favor Brexit—see this as a catastrophic option that could lead to doomsday scenarios like food shortages, troops on the street and a lack of essential medicines.

Many MPs, including opposition leader Jeremy Corbyn, have been trying to convince May to rule out a no deal Brexit, with Corbyn refusing to consult with her until she does so. Such is the febrile nature of British politics that this has, in turn, led to accusations that he’s held the country hostage to secure his ulterior motive of sparking a general election.

Despite this, Parliament will consider some amendments that rule out the possibility of a no deal Brexit. Their chances of success are minimal, however, as many MPs believe its essential to keep the threat of no deal on the table as a negotiating weapon.

Extendable Brexit

One of the most-watched amendments has been tabled by backbench Labour party MP Yvette Cooper. It would guarantee a vote in Parliament to extend Article 50—the clause of the European Union treaty that allows member countries to withdraw—for a period of three to nine months to allow more negotiating time. The vote would be triggered if May fails to reach an agreement with the EU by the end of February. In effect, this amendment would postpone the chances of a no deal Brexit. Initially, it looked like support for this amendment would be strong, but on Tuesday morning it appeared to be waning.

Under an EU Court ruling late last year, the U.K. government is also at liberty to unilaterally revoke Article 50, ending the Brexit process altogether. May has taken that option off the table, saying it would betray voters.

The Brady Bunch

May is putting her support behind the “Brady amendment,” a plan put forward by backbench Conservative MP Sir Graham Brady, which seeks to replace the ‘Irish backstop’ — essentially, an insurance policy against the return of a hard border between Northern Ireland and Eire — with “alternative arrangements”. If the Brady amendment is passed, it would mean that negotiations with the EU would have to be reopened. The wording of the amendment is also thought to have been kept deliberately vague to allow these negotiations to remain as open as possible.

The EU

All of the parliamentary debate and compromise over Brexit may be moot. The EU has stood firm on the positions it negotiated in good faith with the U.K. and has said that it will not renegotiate the withdrawal agreement as it stands.

So, while May hopes to reopen negotiations on how the U.K. leaves the bloc on March 29, the EU may very well refuse to make any amendments. The only choices for Britain then? No deal, or no Brexit.