"We were never really in that game," Tom Wood said in his post-match interview, an honest assessment of the Saints, whose display was far from those of previous weeks. They certainly did not help themselves by conceding early, then immediately after scoring themselves, and again right at the end of the first half.
Things started badly. The Scarlets backs have been their headline makers, but their first try - with just two minutes on the clock - had its foundations on a solid scrum.
Gareth Davies broke from the base, and though the ball hit the deck it came off Liam Williams's boot and the full back beat George Pisi to the touch down. Rhys Priestland landed the conversion from in front of the sticks and as with Thomond Park a week earlier Saints were facing an early seven-point deficit.
They remained under the cosh as the Scarlets continued to ask questions of the hosts across the field. But the green line held, slowly began to turn the screw, and when Ryan Lamb kicked a penalty the nerves started to settle among the sold out crowd.
But within 60 seconds the cheers had turned to boos with a controversial seven pointer from the Scarlets.
The replays had the Franklin's Gardens crowd incensed from both an apparent offside at the kick off and then a knock on from a red-clad hand in the attempted collection of the ball. Regardless, play had been waved on and Aaron Shingler had the try.
That said, the Saints weren't helping themselves either, with a few sloppy passes and fumbles handing possession back to the Pro 12 outfit. Lamb, who continued his perfect record off the tee to bring the Saints back into the match at 9-14 suffered from seeing a long kick bounce over the dead ball line, too.
But the mistakes kept on coming, and one a couple of minutes from time proved very costly indeed. Priestland sent up a towering bomb and while Pisi seemed to have it covered he fumbled the catch to George North. The covering tacklers tried their best, but eventually the numbers ran out and Matt Gilbert - who had come on for the previous try scorer Shingler eight minutes earlier - crashed over by the sticks.
Priestland ensured that the Saints had an even tougher job on their hands by landing the conversion.
The first 10 minutes of the second half were scoreless, but there was plenty of action. The Scarlets came closest to the try line and once again dominated possession and territory. By contrast Saints were still looking disjointed, forcing passes and only the once getting over the gain line courtesy of a powerful James Downey break.
There was another neat-looking set of rucks up the short side in front of the Tetley's Stand, but this was brought to an end by an accidental offside, and as the hour approached and the Scarlets set up a powerful maul a fourth Welsh try looked likely.
It was ultimately stopped and Priestland was forced to send through a grubber which Lee Dickson cleared up.
But you could tell things weren't going the Saints' way in the passages of play around the hour mark. Firstly Vasily Artemyev was errantly called as stepping on the touchline. Then Ryan Lamb hit the post with a penalty immediately after Johnathan Edwards was yellow carded. And to rub salt in the wound, although Saints recovered Lamb's kick the play broke down almost immediately, the ball was hacked upfield and Priestland was over for the bonus point try.
With 20 minutes to play and the Scarlets having built a 19-point lead that seemed to be that. But somehow Saints managed to salvage something out of the contest.
Pisi redeemed himself by barrelling over for his first Saints try, and then with seconds left Wood was over between the sticks. Lamb converted both and in receiving the kick off Saints had the opportunity to complete their own miracle comeback.
It wasn't to be, however, and almost inevitably it was a fumble which brought about the final whistle.
REFEREE Peter FITZGIBBON
2mins SCARLETS Try L Williams Con Priestland 0-7
18mins SAINTS Pen Lamb 3-7
19mins SCARLETS Try Shingler Con Priestland 3-14
22mins SAINTS Pen Lamb 6-14
32mins SAINTS Pen Lamb 9-14
38mins SCARLETS Try Gilbert Con Priestland 9-21
40mins SAINTS Miss pen Lamb
HALF-TIME SAINTS 9 SCARLETS 21
60mins SCARLETS Yellow card Edwards
61mins SAINTS Miss pen Lamb
62mins SCARLETS Try Priestland Con Priestland 9-28
67mins SAINTS Try Pisi Con Lamb 16-28
79mins SAINTS Try Wood Con Lamb 23-28
FULL-TIME SAINTS 23 SCARLETS 28