From deciding whether to order a nuclear strike to choosing his cabinet, the next few days will be a whirlwind of momentous actions for new prime minister Boris Johnson.
Mr Johnson will take control of Downing Street after Theresa May holds her final Prime Minister’s Questions on Wednesday.
PMQs is likely to be an unusually convivial Commons affair after a tumultuous three years at the top of British politics, then Mrs May will head to Buckingham Palace to formally resign to the Queen.
And, with split second timing, her departure from the palace will be followed by Mr Johnson’s arrival, as the Queen officially invites him to form an administration as the 14th prime minister of her reign, which began with Winston Churchill in Number 10.
Mr Johnson will accept the Queen’s request to lead a new government and will, in keeping with tradition, “kiss hands” with the head of state – in reality, shaking hands.
The former foreign secretary will then enter Downing Street as prime minister and is expected to make a statement of intent in front of the world famous black front door of Number 10.
Once inside the official residence, Mr Johnson will take one of the most profound decisions that faces any world leader – what to do in the event of a nuclear attack.
Mr Johnson will meet military chiefs and then be left alone to write instructions to the commanders of Britain’s nuclear strike force in the event that the Trident submarines lose contact with London as a result of a devastating conflagration.
The Letters of Last Resort, as they are known, will give orders to the four submarine commanders of Britain’s Trident nuclear fleet on what to do should Mr Johnson be killed and the country destroyed in the face of a nuclear attack.
The prime minister’s decision on whether Britain should retaliate with a nuclear strike, or whether the submarines should head for a neutral port, or some other course of action, will never be made public.
Mrs May’s instruction letters will be removed from nuclear strike force submarines and burned, unopened, when she leaves office.
The moment the newly appointed prime minister writes the letters is said to be profound.
Former chief of defence staff Lord Guthrie said that Tony Blair “went quiet” when he had to write the letters.
In a BBC Radio 4 documentary, Lord Guthrie said: “I think quite honestly, like most prime ministers, he hadn’t given a huge amount of thought to what this really meant. And it is actually an awesome responsibility.
“It really comes home to you that he could, if the circumstances demanded it, create devastation on a huge scale.”
Mr Johnson will face the challenge of putting together a cabinet that will be unusually tricky, given the intense political tensions within the Tory party over Brexit and the refusal of a number of senior figures to serve under him.
The appointments are likely to come in a flurry from late afternoon on Wednesday, and Mr Johnson is expected to address the Commons for the first time as prime minister on Thursday.
Parliament will then rise for the summer recess, which will provide Mr Johnson with some much-needed breathing space, as a by-election in Brecon and Radnorshire next week could reduce the Tory-DUP majority in the Commons to just one.
From Wednesday the new PM will have exactly 100 days to deliver his “do or die” pledge to pull the UK out of the EU by October 31 – deal or no deal.
A keen historian, Mr Johnson will be aware that the shortest-serving prime minister in history was George Canning, who survived 119 days in the top job between April and August 1827.
The decisions Mr Johnson takes in the coming days could decide whether he beats that record or not.