It has been one of the beauties of this series that, before almost every session, someone has commented: "the next hour is vital."
And while it's something of a clich - pretty much every hour in a Test is vital, after all - it's also been true. Despite the final result in Brisbane and despite the head-start Australia took in Adelaide, it has been a wonderfully fluctuating, absorbing couple of weeks.
But it is hard to avoid the feeling that Wednesday's play could go a long way towards defining the result of this Ashes series. In short: if Australia win, the urn will surely be theirs. But if England score the 178 more runs they require for victory, well we might just have a classic series on our hands.
Oh yes, statistically England can still retain the Ashes if they lose this Test. But that would take a victory in Perth, where they have won just once in the history of these encounters, and in reality the chances are tiny.
More than that, though, you wonder how the respective moods of each team will be governed by this result. England, these days, are haunted by the possibility of a whitewash which will loom into sight if they fail here. While Australia will know they have surrendered an almost insurmountably strong position if they lose. To declare and decline to enforce the follow-on? Steve Smith will go down in history for all the wrong reasons.
It's now or never.
There were moments when Australia looked rattled on day four. Moments when Smith looked frustrated with his fielders - his reaction when Josh Hazelwood failed to take a half-chance offered by a top-edged sweep from Mark Stoneman was revealing - moments when he panicked with his DRS decisions, and moments when his snapping at the batsmen looked uncontrolled and emotional.
David Saker, the Australian bowling coach, wasn't joking when he said the charm of the Barmy Army's taunts were "wearing thin" from an Australian perspective. For all the bluster, you wonder how deep the reserves of confidence are in the Australian dressing room.
They remain favourites. In Pat Cummins and Nathan Lyon, Smith has a couple of aces that should still make the difference. To see Cummins beat Dawid Malan, a set batsman attempting only a forward defensive against an old ball, for pace deep in the last hour was to see a reminder of the sort of the top-class cricketer Cummins could have been and could still become. He is a key point of difference between these sides.
England also seem no closer finding a way to tame Lyon. So effectively did he tie down Stoneman and Alastair Cook on Tuesday that each managed only two scoring shots in 17 and 19 balls respectively against him. Stoneman scored three against him and Cook two. It means Lyon not only provides a threat, but is allowing Australia to get away with playing three seamers as he provides time for them to be rested and rotated. England have to find a better way to manage the challenges he represents.
Still, from a point where they were 142 for seven - 300 behind - in their first innings, it is remarkable that England go into the fifth day of this match with hopes of victory. It underlines not just how badly they played over the first two-and-a-half days, but how much character they have shown to claw their way back into the match. It is something they never managed in 2013-14.
That first-innings stand of 66 for the eighth-wicket between Chris Woakes and Craig Overton might yet be key. It wasn't just that it closed the gap a little, it was that it forced Australia to bowl more overs and might well have convinced Smith not to enforce the follow-on. Deciding - not unreasonably - that his three-man seam attack required a rest, he gave England the opportunity to bowl at his side under lights on day three. It might, just might, be remembered as the turning point of the series.
If it is, England will have, in large part, James Anderson to thank. Taking the opportunity offered by a moving ball, he finally claimed that long-awaited maiden five-wicket haul in his 15th Test and 30th innings in Ashes Tests in Australia. It took his overall Test bowling average in Australia to 36.21, though if you exclude that first series - in 2006-07 - it falls to 31.17.
It was noticeable that here Anderson bowled two marathon spells. The first, on the third evening, lasted 11 overs, while the second, on the fourth morning, lasted eight. It rendered the decision to give him just three with the second new ball in Brisbane, with the game there for the taking, all the more puzzling.
It is an oddity and irritation of a team sport that personal milestones - hundreds and five-wicket innings in particular - continue to disproportionately influence how careers are judged. But Anderson claimed three four-wicket hauls on the way to helping England win the 2010-11 Ashes, including a memorable spell in Melbourne in which he dismissed Michael Clarke, Michael Hussey and Smith on the way to Australia being bundled out for 98. To suggest this performance was better seems perverse. Equally, Woakes' four wickets here - a haul that included David Warner and Smith - was arguably just as good and just as important.
"Anderson should have had nothing to prove for years. Not since he was the highest wicket-taker in the 2010-11 Ashes"
In truth, Anderson should have had nothing to prove for years. Not since he was the highest wicket-taker in the 2010-11 Ashes. Not since MS Dhoni rated him "the difference between the sides" in India in 2012. Not since he bowled magnificently in the UAE without fortune or reward in both 2012 and 2015. From May 2010, he averages 30.32 in away Tests. Since the start of 2015, that figures is 28.19. Those who aren't convinced by him yet - despite those 500 wickets and the testimony of a generation of Test batsmen - never will be.
His record would be better still if he were a more selfish player. Were he the sort who looked to protect his figures he would not have been bowling when George Bailey thrashed him around Perth in 2013-14 and he would not have tried to rush his way back from injury for an India tour towards the end of 2016 that never offered him much more than pain.
From that point, however, when many of us were writing the career obituaries, he has performed better than ever. Since July, he has taken 47 Test wickets at a cost of 15.82 apiece. He has lost some pace, yes, but he has rarely bowled with as much sustained skill or control.
"People keep telling me I'm about to retire, so I want to show them I've still got wickets in me," he said after play on Tuesday. "As long as I'm bowling well and doing myself justice on the field I'm going to keep playing. I absolutely love this game."
A noticeable aberration occurred in the first innings here. Anderson was as responsible as anyone for squandering the first new ball and he knows it.
"We were really annoyed with the way we played for the first couple of days," he admitted. "We didn't do ourselves justice in those two innings with the ball or the bat. There was a lot of frustration. So we wanted to show what we could do. We saw it as an opportunity to show we've got some fight and character in our team. And we've done that.
"Victory would be huge for us. Both in terms of what it would do for our confidence as a team and getting us all-square in the series.
"And it would potentially have an effect on them. For a team to be in such a commanding position and to lose that . It's very rare that a team declares and loses a game. So if we can get a result it would be huge for a number of reasons."
Indeed it would. All the talk, all the planning, all the years of work come down to this.