There he was at No 11, coming out with his side in turmoil at 117-9 having lost five wickets for just nine runs. Sure he had a first-class average in the 30's, but this wasn't state cricket Down Under, was it?
But if England had no clue of his talents with blade in hand at that stage, they certainly did 101 deliveries later. Agar not only closed the gap with a barrage of boundaries, he pushed Australia into a first-innings lead that had looked so unlikely when James Pattinson was ninth man out.
Clearly Agar has talent; he was solid in defence and comfortable against the short ball, hardly a surprise for a man raised on the bouncy pitches found in Perth. He admitted afterwards he batted eight for Western Australia.
His straight six off Graeme Swann was an impressive blow, as too the hook over fine leg that cleared the rope comfortably. However, it was the Kevin Pietersen-like flamingo flick through mid-on that was the most eye-catching of all, showing here was a player with serious potential.
It was perhaps sad in the end that he fell short of becoming the first man to bat at such a position and make a Test century, and even Agar himself said 'sorry' to his family watching at the ground as he came out to field for England's second innings. He had absolutely no need to apologise.
By the close he was still smiling, while in his interview afterwards he claimed he had taken a leaf out of his little brother's textbook in opting to go out there and "just hit it".
Now, though, the 19-year-old will have to do the job he was picked for - keep England's top-order quiet and contribute a few wickets to aid Australia's hopes of going 1-0 up in the series.
Ashton Agar's innings by the numbers:
1: Became the first debutant to make a half-century from no 11 in the order
3: Became the third no 11 to top-score in an Ashes innings (and the first since 1896).
6: The number of runs Agar had to his name when England appealed for a stumping decision. Third umpire Marais Erasmus gave him not out.
10: The number of first-class matches he had played before being handed his Test match debut. Glenn McGrath presented the teenager with his Baggy Green cap on the first morning at Trent Bridge.
71: Agar's best score in first-class cricket before being added to Australia's squad for the tour of England.
98: Highest score by a debutant at No 11 (previous record 45*), highest score by an Australian No 11 (previous record 61), highest score by a No 11 in Tests (beating the 95 made by Tino Best, also against England)
163: Record final-wicket wicket partnership for Australia against England (previously 127), world record partnership for the 10th wicket (previously 151).