After British MPs rejected all eight alternatives for Brexit, a vote on Friday will refocus on Theresa May's plan. The plan has been agreed to by the EU, but has failed to pass in parliament in previous two attempts. 

However this time, May has changed her strategy. MPs will not be asked to vote on the entire plan, but just a part of it. 

Her plan consists of two parts: a legally-binding withdrawal agreement and a non-legally-binding, more general political declaration to outline the future relationship between the UK and the EU. 

The withdrawal agreement sets out the details regarding terms of the divorce. This includes how much money the UK must pay to the EU as settlement, details of the transition period, citizen's rights, and the issue known as the "backstop" - measures to prevent a hard border returning to the island of Ireland. May is asking MPs to vote only on the withdrawal agreement this time, and leave longer-term aspects for the next few weeks. 

How is tonight's vote going to impact the next steps of Brexit? First, today is March 29, which should have been the day the UK officially left the EU, at least according to the initial arrangement. But because parliament rejected May's plan in the previous rounds of voting, the EU last week agreed to delay Brexit. But that extension, through May 22, is only good if the withdrawal agreement is approved this week. So, tonight's vote is parliament's last chance to secure this extension. 

Another crucial aspect of tonight's vote remains May's promise to resign from the prime minister's post if MPs approve her withdrawal agreement. Interestingly, even if May resigns, she would continue to do her job until a new leader is elected. 

However, if the vote fails tonight, the UK would only have until April 12 to come up with an alternative or face a chaotic no-deal Brexit, which means there would be no transition to soften the shock to UK's economy. 

What adds further to the complexity is that if none of these outcomes is favored, Brexit could be further delayed and the UK would have to take part in the European parliament election in May. 

Another referendum could be held to push the situation back to the starting point, or an early general election could be called to reset everything. 

(Shang Jianglong contributed to this story.)