Agar made 98, two runs shy of what would have been one of the most astonishing centuries in the history of Test cricket.
"This is unbelievable, its surreal," Agar’s brother William told the Australian just minutes after falling two runs shy of a hundred.
“Ashton is really chilled but I know he is happy at making such a big score. I know he is pretty excited."
Wesley Agar said when Ashton went for the boundary shot to bring up his century, his stomach was in turmoil for his big brother.
"I thought he was going to get the boundary, and then I thought he (Swann) might drop it, I was hoping he would anyway.
"But to be honest, I just wanted him to score one run and not get a duck so its been the most exciting thing ever."
If the rest of the nation was disbelieving, think of the panicked preparations the Agar family had to get to Tullamarine to catch a flight to London to watch their son's shock debut for the Australian XI in the opening Ashes Test.
At a time when Sonia and John Agar thought their eldest son, 19-year-old Ashton, was at Trent Bridge bowling in the nets, ostensibly spinning some balls for the likes of Phil Hughes, David Warner and Michael Clarke for batting practice, they received a late-night phone call.
Ashton was on the phone, not to regale them with an update that hot weather was beating down in Nottingham, but to quietly pass on some rather good news. In his understated way, he told them he was in the Test team. He, of course, was "over the moon" having received the best dispatch of his young life from Darren Lehmann and Rod Marsh while in the nets.
"Dad answered the phone and I just said 'I'm playing', Agar said, noting that his father's response was somewhat initially more demonstrative. "He was rapt, really. Actually, he was then a bit lost for words."
Cue much excitement in the Bentleigh, Melbourne, household late on Monday night. But it had to be kept a secret. "We had to grab a few things and we just made it. We landed at 6.30 this morning," Sonia Agar said.
Just three hours after the long flight, they were on the fringe of the historic ground watching their son receive his baggy green cap from none other than Glenn McGrath.
Sonia, of course, shed a quiet tear, but so too did her new Test-playing son.
Dad John was beaming and Ashton's little brothers Wesley, 16, and William, 17, looked on with pride and a dash of envy.
John Agar, a useful cricketer himself who has encouraged his three sons to excel at backyard cricket, said Ashton had done the hard work required to succeed.
"When he was young, he played top order for the school at a young age and he is able to handle it -- he deserves a shot.
"He can take wickets and score a few runs and he's cool, a pretty cool character. He's calm and doesn't get flustered."
Certainly Agar will have to draw upon that detachment -- and what he describes as his "youthful enthusiasm" -- to snare a couple of wickets in the second innings, even if the Dukes ball and its slightly more pronounced stitching feels foreign.
"There was a bit of bounce today and a bit of grip. It was quite a dry wicket and it should wear up a bit later," he analysed.
Agar, a tall, attacking finger-spinner, was fast-tracked by Lehmann after breaking through into the West Australian Sheffield Shield team just six months ago specifically to attack England's right-handed batsmen and was named ahead of Nathan Lyon. Agar had made the move across the continent from Victoria because he felt there were more openings for a spinner in WA.
Such was the rapidity of his Trent Bridge appearance, he wasn't added to the Ashes squad until after his Test selection.
Agar said of the most unexpected experience: "It was awesome walking into the (Trent Bridge) ground. I did a circle and looked at the crowd with the ball in my hand and saw my family. I was a little bit nervous but once the ball was out of my hand it was fine."
Agar bowled seven overs for one maiden and had figures of 0-24 after the first day's play. Now that the first day's excitement has elapsed, this rap music lover and deferred law student at Murdoch University said he felt much more comfortable.