Ireland's Grand designs hinge on Sexton
This is perhaps the most significant few hours of Joe Schmidt's time with Ireland so far. Until next week at least.
By 7.0 tonight, his team could be Six Nations champions for the third time in five years thanks to an Irish record 11th successive victory.
Win, and they'll have a chance to join the men of 1948 and 2009 in the pantheon against England on St Patrick's Day.
Lose, and there will still be plenty to play for in Twickenham next weekend.
Yet this already epic campaign has been building towards something special and, while a title shoot-out is not to be sniffed at, the Grand Slam is everything until it can no longer be achieved.
There is also a need to stop ceding ground to a Scotland team who Ireland meet in Yokohama in just over 18 months' time in an opening World Cup match that could have major ramifications on both teams' tournaments given the All Blacks probably lie in wait for the losers - presuming they can recover to beat hosts Japan.
Coached by a clever, highly regarded young tyro in Gregor Townsend, they arrive in Dublin full of belief and in search of a big away scalp to cement the feeling that they're making progress and to keep their own dreams of a first Six Nations title alive.
Under Vern Cotter, the perennial underachievers got back on track and narrowed the gap on their rivals from across the North Channel to the point where they finally tasted victory in this fixture last season.
That was at Murrayfield. Now, they must prove themselves away from Scottish soil in this tournament. They beat Australia in Sydney last June, so the idea that they don't travel at all is something of a myth.
Rather, it was their prime motivating factor as they made the short flight to Dublin after conducting their Captain's Run in Edinburgh yesterday.
Indeed, they are not short on carrots as they get used to their surrounds.
Ireland lead England by five points after three games, but the Scots are one behind Eddie Jones' side with a potentially boot-filling trip to Rome on the final day.
If they can overturn the odds today, then they'll hold out some hope of sneaking a title on St Patrick's Day.
Imagine the deflation at the Aviva Stadium if they manage it.
It's perfectly plausible that they might, they possess some of the most potent strike-runners in the northern hemisphere and are strong in Ireland's area of greatest vulnerability: the wide channels.
However, this is an Irish team that seems to have forgotten how to lose despite the white-knuckle nature of their Six Nations ride to date.
Big finishes against France and Wales has filled them with belief that they can get out of tight spots if needed, while their overall dominance in a five-try win over Warren Gatland's side showed that despite the regular cribbing about their lack of attacking cutting edge they can put teams to the sword.
They have thrived in pressure moments, but the stakes are rising by the week and it remains to be seen if the young players, who have shown a remarkable aptitude for life at the top level, can handle the pressure that comes with history.
The old heads will lead by example.
Rob Kearney and Rory Best are survivors from the Grand Slam season, while Johnny Sexton, Conor Murray, Cian Healy, Devin Toner, Peter O'Mahony, Seán Cronin, Jack McGrath and Iain Henderson have Six Nations medals in their back pockets. Keith Earls would too but for his injury luck.
Alongside them is a precocious new breed of players, most of whom are yet to lost a Test match.
Dan Leavy, Bundee Aki, Jacob Stockdale, James Ryan, Andrew Porter, Joey Carbery and Jordan Larmour have no idea what it's like to finish on the losing side.
Sexton, as ever, is the key to it all.
His radar was off against Wales, but there was nothing wrong with the rest of his game despite the gluteal issue that has been troubling him since the Italy game.
His all-action approach two weeks ago had his coach wincing, but he accepts you can't change the warrior nature of a No 10 at the top of his game.
"Do I wish he was more judicious? Sometimes I do," he said of the play-maker putting himself in harm's way..
"I remember sitting in coaches box with (ex-Leinster forwards coach) Jono Gibbes at one stage and Jono saying, 'Would you ever get Johnny to stay out of that stuff, leave it to my boys. We will look after that'.
"He's a competitor. The other players love that about him, because it gives him licence to lead.
"When you put your own hand up, it's pretty easy to lead others and he does a super job of it."
When you have a player who can deliver a crisp, clean, cut-out pass for Jacob Stockdale who is also able to drill a powerhouse back-row like Ross Moriarty behind the line in another, you're on to a winner.
Keeping Sexton fit and motoring is key.
Opposite him is a man who can unlock defences in a heartbeat in Finn Russell. The only question is which defence he'll choose, the opposition's or his own.
The Racing 92-bound fly-half is a brilliant talent with a superb passing range who can dominate or disappear depending on his mood.
Two weeks ago, he responded to a disappointing start to his tournament with an excellent display against England. Schmidt was impressed.
"The whole energy of it, the pressure they put on the breakdown," he said of the Calcutta Cup win.
"I think you saw John Barclay at his very best, ably supported by Hamish Watson, who is super, Ryan Wilson, Jonny Gray, they were very big there.
"Out wide, I think Finn Russell was back to pulling the strings, as we know he is capable of. His pass out to Huw Jones at one stage was spectacular.
"They are such a threatening team, when you think there is nothing on, they conjure something up and that's a real danger for us.
"We've got to make sure that we don't assume anything, that we make sure we're nailed on.
"We think there is probably further improvement in them the way they have stepped up from Wales to France to England and they're right in the mix.
"They know if they come here and get a win that they are right in the championship. They can go to Rome knowing that if they can get a win there, they will be four of five and potentially that's going to give them the points to win the championship."
It is a challenge an improving Irish back-row will relish.
Of the three, only CJ Stander was present for last year's loss but the footage of Ireland standing off the Scots will have stung them in their review sessions this week.
For a team who pride themselves on breakdown accuracy, the performance was well below the level they expect of themselves and allowed Russell to play, as Rob Kearney memorably put it, "in a dinner jacket".
Given the threats Scotland possess, that would be silly.
Focus on their width and Jones can cut through the middle. Spread yourself too thin and Jonny Gray, Barclay and Ryan Wilson can take it up the guts.
The trick, as it is often is with Ireland, is to starve them of possession.
Last season, they dominated the scrum and they'll back themselves to get the upper hand there again. Devin Toner's selection is at least a nod to the lineout troubles they experienced last year, while their accuracy at maul and in the tackle must be much higher.
Retaining possession through the phases and building pressure is the Irish way.
Garry Ringrose's involvement adds an extra layer to the Irish attack. He is light on minutes this season, but his clever running style and sheer pace off the mark will ask questions of the Scots.
Defensively, the midfielder needs to find his feet early. His form for Leinster in this department has been scratchy and he is in the middle of an unfamiliar backline, but he must adapt quickly and get to grips with the tricky Jones.
As the outside centre, his decision-making and communication will be key. Memories of Stuart Hogg dancing in the wide channels will be foremost in his mind this morning, but he has the capacity to shut down the space.
Scotland have threats, but Ireland will look to squeeze them.
They have the better tight-five and the superior half-backs. Andy Farrell will demand more of a defence that has been leaking tries at a worrying rate, while the attack has room for growth despite a total of 13 tries in the last 80 minutes.
Ireland can't expect to score five tries every week though, so tightening up is a priority.
Their five-point cushion means they don't need to chase a bonus point, but Rory Best will be fully aware of the scenario and if they get a sniff of the chance to heap further pressure on England then they'll go for it.
To have the title in the bag as they head to London is the dream scenario.
It's time for Ireland to make it the reality.