Stacey Jones jinked through the St Helens defence with 35 seconds remaining to snatch a 26-22 victory for Catalans Dragons which appeared unlikely for much of the second half.
With 15 of Saints� likely Challenge Cup final line-up left at home, it was a largely unrecognisable team that took the field for the visitors, with 13 players aged 21 or under.
The youngest of those, 18 year-old Dean McGilvray, got an early taste of the size of the task in hand when he was knocked backwards by the power of French forward Jamal Fakir.
By then St Helens were already behind, with Lionel Teixido having crashed through the defensive line after just three minutes.
Pacy winger Justin Murphy added his 24th try of the season to double the Dragons� advantage and it seemed a sporting landslide could take place in the shadow of the Pyrenees.
Scott Moore scored from dummy-half to respond for the visitors but Murphy added his second moments later after Saints conceded consecutive penalties then Gregory Mounis scored the Dragons� fourth try for a 20-6 lead.
Jones had been pulling the strings for the Dragons as the young defence struggled to cope with the threat of several runners but the New Zealand international was having less success with the boot, failing with two conversions.
Instead it was Saints that profited from an astute kicking game as they got right back into the game.
Craig Ashall�s grubber kick was missed by full-back Thomas Bosc and Craig Littler was on hand to touch down, then Gareth Langley out-jumped Bruno Verges for Matty Smith�s pinpoint kick.
Ashall scored his second try and that looked like it would be the only score of the second half as the clock ran down.
The Dragons� task was made harder by the 74th-minute dismissal of Jerome Guisset for a high tackle on Nick Fozzard, but Jones had the final word to snatch victory.
Catalans Dragons (20) 26. Tries: Teixido, Murphy 2, Mounis, Jones. Goals: Jones 2, Bosc.
St Helens (16) 22. Tries: Moore, Littler, Langley, Ashall. Goals: Langley 3.
Catalans Dragons: Bosc, Murphy, Sadaoui, Hughes, Verges, Wilson, Jones, Beattie, Teixido, Guisset, Fakir, Hindmarsh, Mounis. Replacements: Chan, Griffi, Rinaldi, Gossard.
St Helens: Hardman, McGilvray, Gilmour, Littler, Langley, Ashall, Smith, Fozzard, Webster, Leyland, V Anderson, Clough, Graham. Replacements: Bannister, Rigby, Hodgkinson, Moore.
Referee: Ashley Kline (RFL)