I was impatient to get going. Plans had been made. I knew what needed to be done, but the weather was against us. From the window of the Royal Air Force jet, all I could see were the heavy clouds beneath us as we circled over Scotland. Thick fog had rolled in around the airport in Aberdeen, preventing planes from landing, so for the moment I was stranded in mid-air. As a woman in a hurry, the delay was frustrating. My mind was already turning over the huge number of things I needed to do back in London once I took over as Prime Minister of the United Kingdom. But before all that, I had an appointment with Her Majesty the Queen, and we were now at risk of being late or not getting there at all. At last, a gap in the clouds appeared, and the pilot managed to get us down on the ground. Another bumpy landing.

Boris Johnson, my predecessor, had flown up ahead of me on a different plane to see the Queen and officially tender his resignation as Prime Minister. His idea that we should fly together had been vetoed by the Cabinet Office. I assume it was too much of a security risk to have the outgoing and incoming Prime Ministers on the same plane.

On arrival in Aberdeen, the plan was for me, my husband Hugh and my principal private secretary Nick Catsaras to transfer to a helicopter for a short flight to Balmoral Castle, said to be the Queen’s favourite residence. The fog made this impossible so instead we set out by road, adding yet more time to our journey. Our small convoy eventually arrived at the castle, where we were welcomed by the Queen’s private secretary Edward Young and shown inside. Then, alone, I was shown into Her Majesty’s drawing room.

The Queen, at the age of ninety-six, seemed to have grown frailer over the previous year, but she evidently was determined to carry out her constitutional duty of appointing the Prime Minister in person, as she had for each of my thirteen predecessors. I was told in advance that she had made a special effort to be standing to greet me, and she gave no hint of discomfort. She was as resolute, determined and charming as ever. Although I’d seen her at various Privy Council meetings and events, this was only my second one-on-one audience with her. On the previous occasion, after I had been removed from a different job in the government, she had remarked that being a woman in politics was tough.

After I had accepted Her Majesty’s invitation to form a new government, we spent around twenty minutes discussing politics. She was completely attuned to everything that was happening, as well as being typically sharp and witty. Towards the end of our discussion, she warned me that being Prime Minister is incredibly ageing. She also gave me two words of advice: ‘Pace yourself.’ Maybe I should have listened.

Once we’d finished our conversation, Hugh joined us for a few minutes. She asked about our daughters and made some jokey observations about our new living quarters at Downing Street. We left with Her Majesty telling me she looked forward to our speaking again next week. I had no idea this meeting would be both our last and her final formal engagement as monarch.

I left Balmoral as Prime Minister, and we began the trip back to London. Once again, the weather frustrated our plans as torrential rain poured down. I was amazed by the number of people who showed up to film the car and watch my every move. I was due to give my first speech outside 10 Downing Street before going inside but an indoor option was prepared instead as the skies darkened. Convinced that fate was on my side and the weather would clear, I insisted on circling round in the car.

Eventually, the moment came.

I went straight from the car to speak at the lectern in front of my new home, telling the country: ‘Together we can ride out the storm.’ Having delivered this optimistic forecast and posed with Hugh for the obligatory photographs, I went inside to begin work. There was a lot to do: I had the Cabinet to appoint, my first Prime Minister’s Questions the next day to prepare for and a major announcement on our support for people’s energy bills the day after that. We had prepared a plan for the first 100 days in office – and there was no time to lose. There was also no time to stop and reflect. Some people have asked how it felt to win the leadership election of the Conservative Party and thus become Prime Minister and step over the threshold of No. 10. What was going through my mind? The truth is, the whole experience, from the moment Boris resigned, had felt like a roller coaster, during which I was constantly in performance mode. I was moving from event to event, meeting to meeting, knowing that at this early stage I had to get everything right.

What actually came next, of course, was a profound shock that would reverberate around the world.

The civil service and royal officials had been quietly making plans for the Queen’s funeral and the accession of the new monarch for decades. ‘Operation London Bridge’, as these plans were called, had been worked out in immense detail and tweaked over the years by successive governments in readiness for just this moment. But on a human level, we were utterly unprepared. As I had just seen for myself, the Queen had remained robust, mentally sharp and determined to do her duty. There simply wasn’t any sense that the end would come as quickly as it did.

The first real indication I had of the gravity of the situation was on Wednesday night, the day after I had become Prime Minister. Having appointed my new Cabinet, my new ministers were set to be formally sworn into office, with the Queen joining remotely by video link from Balmoral. As we assembled in the Cabinet Office just before 6.00 p.m. for the meeting, word reached us that Her Majesty would not be available, as she had been advised to rest. That was when the machine kicked into action. My black mourning dress was fetched from my house in Greenwich. Frantic phone calls took place with Buckingham Palace. I started to think about what on earth I was going to say if the unthinkable happened.

The following morning, I was given an update that there were ongoing concerns for Her Majesty’s health and contingency plans were starting to be stepped up, but with no further comment from the palace and no clear idea how quickly things would develop, we had to press ahead with the day’s business. I went mid-morning to Parliament, where we were scheduled to have a debate at 11.40 a.m.

The House of Commons was full of the usual political squabbling. I was set to speak about my government’s plans to tackle energy prices, though I had begun to think about a completely different speech that it would be my duty to give. Not long after I sat down following my speech, Nadhim Zahawi, the Chancellor of the Duchy of Lancaster, entered the Chamber and came to sit next to me. I had spoken to him earlier about his role in coordinating some of the necessary arrangements if and when Operation London Bridge kicked in, so this was clearly not a good sign. He told me we had received news that things were very grave indeed. The palace was about to issue a statement to the media that the Queen was ‘under medical supervision’ and her doctors were ‘concerned for Her Majesty’s health’.

Up to this moment, I had believed concerns might mount over a period of days and weeks, a drama unfolding in slow motion, but I now realised with dread that the news could come in a matter of hours. Members of the royal family were rushing to Balmoral, and the media had recognised the significance of that. I left the House of Commons and headed back to Downing Street.

Later that afternoon, we received the solemn news. Her Majesty Queen Elizabeth II had died peacefully at Balmoral at the age of ninety-six. Despite the preparations that had taken place over the previous twenty-four hours, the confirmation came as a profound shock. After the frenzy of the leadership election and on only my second full day as Prime Minister, it seemed utterly unreal. Amid profound sadness, I found myself thinking: Why me? Why now?

Leading the nation in mourning after the death of our beloved monarch of seventy years was not something I had ever expected to do. I had come into office determined to focus on the British economy, which was heading for a downturn, and to take the tough decisions necessary to stimulate growth and put the country back on the right track. These were challenges I instinctively relished. But coping with the death of the Queen was something altogether different. I had experienced a fair amount of state ceremony and protocol during my time in politics, but in truth, it was a long way from my natural comfort zone. Some Prime Ministers might have been better suited to the soaring rhetoric and performative statesmanship necessary in this historic moment, but I just felt a profound sense of sadness.

Queen Elizabeth II had been a constant in the lives of British people for seventy years. There were few in the country who could remember a time without her. Everything from postage stamps to banknotes were a perpetual reminder of her presence. She had touched the lives of millions. Her calm reassurance and stability had given succour during hard times for the nation. Even until the last, people could appreciate her sense of duty above all else. No other British monarch had...