It was on Friday, during the marathon 12-hour Brexit summit at the PM's Chequers country house, that David Davis realised it was time to resign.
"He presented a strong argument to Cabinet and understood that if he lost that argument and didn't believe in the policy, the natural thing was to step aside," a friend told HuffPost.
Davis wanted to chat through his decision with his wife and his local Conservative Association in his Yorkshire constituency, which he duly did. Having made his mind up, Davis then went to Silverstone race track for the British Grand Prix.
The fact that a German driver won, rather than Britain's Lewis Hamilton, seemed like a neat metaphor for the way May's "soft" Brexit was heading. The outgoing Brexit Secretary felt May's plans were not just unworkable but they would lead to Brussels demanding even more concessions.
He even left the decision late to miss the newspapers front pages, so as not to damage Theresa May Westminster source
Back in London, Davis left his Pimlico home and headed to No. 10 on Sunday night. Over a drink, the Prime Minister's communications chief, Robbie Gibb, tried to talk him out of quitting. Chief Whip Julian Smith also tried in vain to get him to change his mind.
It didn't work. Davis then rang May personally at home in her Maidenhead constituency, announcing that he was indeed resigning. Almost exactly two years since he was brought back from the political wilderness by Theresa May after David Cameron's resignation over the EU referendum result, he was gone again.
Back in July 2016, he sat across the Cabinet table and told May he would make it the mission of his "last job" in government to make her premiership a success. He was keen to reassure her that he had no leadership ambitions of his own. Fast forward to today and he's suddenly presented May with the most serious threat to her political career since she became PM.
Davis decided to make his announcement late on Sunday night, to try to remove any personal animus from the decision. "To be fair, he's been loyal and supportive. He even left the decision late to miss the newspapers front pages, so as not to damage her egregiously," one ally said. But No. 10 knew the damage would be done whenever he timed it, and his resignation has fuelled intense speculation that yet more backbench MPs will submit letters demanding a vote of no confidence in May.