After a few hours of sleep to shake off a night of celebration and an audience with the King, Keir Starmer stepped through the front door of 10 Downing Street for the first time as prime minister on Friday.
When he did, he entered the alternate reality of a man who meets regularly with King Charles III and has ultimate control of Britain's nuclear missiles, all while adjusting to life in a creaking 17th century landmark and trying to balance his work and personal life.
On his first day in office, Starmer will get briefings from senior civil servants about key issues facing the government, receive congratulatory phone calls from world leaders and begin the process of appointing his Cabinet.
Here is a look at some of the other traditions and responsibilities he faces on his first day inside No.10:
Clap for the leader
The first time a prime minister walks through the uber-polished door of 10 Downing Street, household staff and civil servants by custom line the entrance and clap for the new leader and his senior team.
It is Starmer's introduction to the people he will live and work with, most of whom served his predecessor only a few hours earlier.
Salma Shah, a special adviser to former Treasury chief Sajid Javid, described the custom as equal parts nice gesture and strange experience, particularly given that the civil servants know little or nothing about the latest batch of politicians moving into the heart of British government.
"I've often mused over the fact that no one really claps you on the way out whenever you leave your job,'' she said during a briefing about the first days of a new administration sponsored by the Institute for Government think tank.
"So it's nice, but it is also quite odd.''
Nuclear trigger
One of the most sobering moments of any prime minister's first day on the job is the realisation that he now has the ultimate authority over whether to launch Britain's nuclear missiles.
In the UK, this is underscored when the country's top civil servant informs the new prime minister that he has to write "last resort letters" to the captains of Britain's four nuclear-armed submarines telling them what to do in the event of a nuclear attack that wipes out the civilian leadership.
It is a duty unique to Britain, where there is no "nuclear football", the briefcase carrying targeting data and launch codes that accompanies the US president wherever he goes.
The letters are placed on board each of the submarines inside safes that are to be opened only if their captains are certain Britain has been attacked and the country's civilian leaders are dead.
While the letters are destroyed unread when a new prime minister takes office, there are thought to be only four options: retaliate, don't retaliate, use your own judgment, or put your nuclear weapons under the command of the US or Australia, if possible.
This isn't the White House
No.10 Downing Street is as much of a shorthand for Britain's prime minister as the White House is for the US president. But that's about all they have in common.
Behind the famous black door of No.10 sits a warren of interlinked offices, meeting rooms and two residences carved out of three townhouses built in the late 1600s.
With an estimated 400 people working in some 100 offices, the space has become dysfunctional, a workplace consultant said two years ago, recommending that the prime minister's top team move to a modern office space.
"It's clear that 10 Downing Street isn't fit for purpose and much of the muddled decision-making afflicting the government may stem from not having a proper office," Andrew Mawson, managing director of Advanced Workplace Associates, said in 2022.
"No major corporation – or indeed government department – operates from a largely unreconstructed 300-year-old building or has the CEO living above the shop."
One of the first decisions Britain's new prime minister will have to make is whether to live in the two-bedroom apartment above 10 Downing Street, traditionally the home of Britain's leaders, or the more spacious four-bedroom apartment over No.11, formerly dedicated to the treasury chief.
Starmer, who is married and has two teenage children, is likely to follow recent precedent and claim the larger apartment. Other than his predecessor, Rishi Sunak, every prime minister since Blair has picked that option.
No.10 Downing Street is part of a row of townhouses built between 1682 and 1684 by former diplomat and property developer George Downing. The home of Britain's prime ministers since 1735, it has been expanded over the years by linking it to the adjoining properties at No.11 and No.12.
There were problems from the beginning.
In an effort to increase his profit, Downing cut costs. The houses had inadequate foundations for the boggy ground and the mortar lines were drawn on to give the appearance of evenly spaced bricks, according to the government website.
One former resident, Winston Churchill, described Downing Street with his characteristic flair.
"Shaky and lightly built by the profiteering contractor whose name they bear."
Mouser in chief
Before the day is over, Starmer may have his first meeting with Larry the cat, far and away the most famous permanent resident of Downing Street.
Larry, a gray and white tabby who roams the heart of government as if it is his own personal realm — has been a fixture of the residence for more than 13 years, outlasting five prime ministers.
The former stray was brought to Downing Street from the Battersea Dogs and Cats Home in 2011 to help control a rodent problem and he has been "chief mouser" ever since.
Larry has the press corps at his paws, with photographers snapping photos of him whenever the news is slow, or all the time really. Larry even has 843,000 followers on X, formerly Twitter.
So here's the question: After Starmer writes his last resort letters, meets the civil servants and starts to adjust to this huge change in his life, will he make time to scratch Larry behind the ears? And what will Larry think?
After all, prime ministers come and go. But Larry? Well, he had better not be going anywhere says freelance photographer Justin Ng, who is known as Larry's favorite snapper on Downing Street. Perish the thought that Larry might retire!
"I hope Mr. Starmer ... doesn't underestimate the popularity of Larry,'' Ng said.
"Basically, if he wants to stay in power, then Larry has to stay, too.''
