If anyone had suggested three months ago in Grenada, when England crumbled into a heap and were beaten by West Indies, that they would beat the world Test champions 3-0, I would have said they were talking rubbish.
There was no morale around the England team. They were playing tentative cricket.
There was uncertainty about the futures of veteran bowlers James Anderson and Stuart Broad, about whether Joe Root could continue as captain and what would happen if he did resign.
The visiting fans milling around the island were disillusioned and it really did feel like England had hit rock bottom.
To come from that to where they are now, having completed a clean sweep over New Zealand with Monday's win at Headingley, is remarkable.
I was on holiday when Brendon McCullum was appointed England coach last month. It came from nowhere but slapped me in the face what a good appointment it was.
I have dealt with him quite a bit over the years, when he was New Zealand captain and later when he worked in the media, so I got to know what sort of person he is - someone who is ceaselessly positive.
However, introducing that to English cricket could have gone one of two ways. It could have been wildly chaotic and ended in defeat, or gone as it has.
Thankfully for England, it has been the latter. In truth, things could not have gone better.
I do not think McCullum has done very much in his role, but he has made the England players believe in what they can do.
The clearest beneficiaries have been Jonny Bairstow and Jack Leach.
Bairstow is someone who has always felt he has to prove himself.
He is a bit on the fringe of this team socially, but on the eve of this Test was throwing a barbeque for the team and support staff at his house, taking charge of the food alongside Anderson.
That is quite telling. He has been made to feel included by McCullum and captain Ben Stokes, so it is no coincidence how he has responded with the bat, although I doubt even he would have expected a return of 394 runs at an average of 78.80 and a strike-rate of 120.12.
Spinner Leach, meanwhile, produced the best performance I have seen from him on a flat pitch.
Forget the 10 wickets in the match for a moment. More impressive was his consistency and his ability to hold an end in the first innings.
He is another character who needs to feel valued - and it has been very noticeable how much Stokes has tried to encourage him on the field.
Stokes has been keenly celebrating Leach's wickets, patting him on the back and pulling his cheeks.
That has clearly had an impact on Leach, while Stokes looks to be loving the captaincy.
I hated the way he batted in the first innings, holing out to mid-off after earlier charging down the pitch to his first ball. There was no need - he needs to give himself a chance to get in - but otherwise he has had made very good start as captain.
His field settings have been excellent, like in New Zealand's second innings when Michael Bracewell hit Leach for six as the tourists' lead began to move towards 300.
Ninety-nine captains out of 100 would have pushed the fielder back in that situation to protect the runs, but Stokes didn't, Bracewell was enticed to go again and was caught.
Yet the big thing about Stokes is he has much more depth to his character than people think.
He has a soft centre and a strong relationship with his team-mates in a nurturing way. You could see that in the way he has managed Leach, whose issues have been well-documented.
The way Stokes has started as captain is one of many positives - the form of Bairstow, Leach and Ollie Pope a few others - but, for me, the biggest has been the emergence of seamer Matthew Potts.
He looks a terrific find. I could not have been more impressed.
Coming into the series, we had those familiar doubts over the fact his wickets had come for Durham in the second division of the County Championship. Would he be able to make the jump to the highest level?
The answer is absolutely yes.
The only query remains opener Zak Crawley, who ended the series without a half-century.
His second-innings knock on Sunday, when he made a frantic 25 from 33 balls, was mixture of sublime and the horrible.
Crawley has been given another opportunity for the Test against India starting on Friday, and he has to give himself a chance by playing his way into his innings.
You can see a scenario where England catch India cold at Edgbaston amid their wave of momentum.
There will be ups and down for England - you cannot keep playing cricket in this way without it sometimes going wrong - but they are on to something.
I got a message from former Australia captain Steve Waugh the other day.
He was telling me how he hoped England continued with this approach because it would give us a compelling Ashes series next summer.
If England have caught Waugh's attention down under then it can't be too bad.
Jonathan Agnew was speaking to BBC Sport's Matthew Henry.