They had been humbled 40-7 by Leinster at Franklin's Gardens on the previous weekend and few gave them a prayer of avoiding a similar defeat at the Aviva Stadium.
But Jim Mallinder's men turned up in Ireland - and how!
Some incredible resistance allied with some ruthless execution allowed Saints to steal the show in front of the stunned home crowd.
Saints were under huge pressure at 13-9 up late on, but Jamie Elliott pouched the ball, sprinted almost the length of the field and scored to put the seal on a memorable success.
It was a night that will live long in the memory of every Saints supporter who witnessed it.
And ahead of this weekend's sixth anniversary, which, aptly, Saints will celebrate by visiting the Aviva Stadium to face Leinster again, we have taken a look back at our man Tom Vickers' tweets - and some replies to him - from December 14, 2013 in Dublin.
4.40pm: Calm before the storm at the Aviva Stadium now. Superb press room curry consumed. Just 80 minutes until the big kick-off.
5.05pm: Late change for Saints tonight: Dom Waldouck OUT, Tom Collins IN. George North moves to 13. Leinster change: O'Brien OUT, Jennings IN.
6.09pm: Very different start to last weekend. Burrell sent North over with a nice pass and Myler made the conversion. Leinster 0-7 Saints.
6.11pm: From @ItsMeRups to @WheresTommyV: Here's hoping @SaintsRugby do what Leinster did last week? 2 tries in first 8 mins? #ItsAStart
6.13pm: Leinster have responded well to that early setback and Ian Madigan reduces the arrears with a penalty. Leinster 3-7 Saints after 11 minutes.
6.33pm: Huge spell of pressure from Saints but Leinster just would not buckle. Massive roars of approval from the home fans as they hold out.
6.47pm: HALF-TIME! Leinster 3-7 Saints. North try for Northampton, who have been unrecognisable from last week's horror show.
6.48pm: Stephen Myler's missed kick before the break prevented Saints from going in seven points up. They would not have been flattered by that.
6.50pm: From @TebbsG to @WheresTommyV: Terrific first half from Saints but gap not big enough. Leinster sure 2 hit back after break.
7.06pm: Some huge tackling from Saints to stop Leinster scoring a try, but they do have a penalty, which Madigan kicks. Leinster 6-7 Saints.
7.14pm: More big defending from Leinster sees Saints lose possession five metres out. Immense frustration.
7.30pm: Not much respect for the kicker here as whistles greet Myler's penalty attempt. It doesn't matter. He makes it. Leinster 6-10 Saints.
7.38pm: Dylan Hartley penalised, Madigan kicks. One point in it now. Leinster 9-10 Saints. Same score as at Ravenhill last year...
7.39pm: From Mrsfeetup to @WheresTommyV: Every single bit of me crossed #painful
7.41pm: Chorus of whistles has an effect on Myler this time as he shanks his penalty attempt. Leinster 9-10 Saints with FIVE minutes to go.
7.44pm: Unbelievable! Kahn Fotuali'i with a left foot drop goal! Saints lead by four with less than two minutes to go. Leinster 9-13 Saints.
7.48pm: TRY, TRY, TRY! Jamie Elliott.
7.48pm: SENSATIONAL! Saints have won at the Aviva Stadium.
7.50pm: FULL-TIME! Leinster 9-18 Saints. Words can't really do that display justice. Sensational, incredible, gargantuan, heroic. Take your pick.
7.51pm: @TebbsG to @WheresTommyV: Have never seen @saintsrugby tackle so hard so consistently, so determinedly. Absolutely awesome display!!
7.51pm: From @r_mulligan1 to @WheresTommyV: Memories of this time a year ago #bigturnaround #greatgame
7.58pm: From @Cheapbloke to @WheresTommyV: Have a good night Tommy. Enjoy the Guinness.
8.31pm: Leinster 9-18 Saints: My match report from a stunning success for Northampton here in Dublin bit.ly/hugesaintswin
8.44pm: Jim Mallinder has told the Saints players to get on the beer after tonight's win. Full reaction from Dublin here: bit.ly/2YEIZiS
9.35pm: When Saints win in Dublin and your hotel is on the main party street. Yeah, that feeling. Have a good night wherever you're celebrating.