Just when you thought it couldn’t get any worse at the Gardens...
The all-conquering England captain comes back and is sent off six minutes into his return to club rugby.
JJ Hanrahan, one of the most creative players in the squad, gets another injury.
Leinster run in five tries.
Saints fall seven points behind the Irish team in Pool 4.
And they haven’t even got a strong standing in the Aviva Premiership to fall back on as they are now 11 points off fourth-placed Leicester Tigers.
Oh yes, and they could be facing a punishment from Premiership Rugby and the RFU for their handling of the George North incident.
These are sobering times at Franklin’s Gardens, to say the least.
Supporters who purchased season tickets at the start of the season could never have imagined the anguish they would feel prior to Christmas.
They would have hoped for moments that made them get up off their seats.
To be fair, they have had them, but not for the right reasons.
Rather than jumping to their feet in excitement, they are jumping to their feet and heading out of the ground before the final whistle.
Some will say real supporters stay until the end.
But when there is no way back and the only thing ahead is a traffic jam, you can’t blame people for leaving.
They don’t want to stay to watch Leinster twist the knife with more tries.
They don’t want to stay to watch George Pisi add his name to the list of recently sin-binned Saints players.
They don’t want to watch the Leinster flags waving furiously, just as they were when the same team won 40-7 at the Gardens back in December 2013.
They don’t want to stick around to listen to the Irish club’s songs belting out of the Rodber Suite.
They don’t want to watch their heroes look painfully punctured.
Why would they?
It really was a soul-destroying night in Northampton.
And perhaps it was made even worse by a fleeting glimpse of what Saints supporters used to witness.
Fleetingly, the intensity of the double-winning season - when the clubs last met - was back.
Fleetingly, Saints stood up to their opponents and defended with real desire.
For a few minutes before the break, the defensive defiance the club showed during the win in Dublin three years ago was present.
And for a few minutes after the break, Saints played some fizzing attacking rugby, resulting in a superb Ahsee Tuala try.
But those moments of promise didn’t last.
It was like a chef standing in front of a starving man, giving him appetisers only to tell him the main course would never arrive.
Eventually old problems resurfaced - one in particular.
Hartley has done his club and himself proud during his time as England captain.
He has steered the national team to a Grand Slam, a series whitewash in Australia and a perfect 2016, with 13 wins from as many matches.
And Saints fans will have been so proud to see him lift the Cook Cup at Twickenham before the big derby game at Welford Road kicked off last weekend.
But they, and he, will not be proud of what he did on Friday night.
Because the red mist moment that saw him strike Sean O’Brien with a swinging arm was exactly what his club didn’t need.
They needed his shrewd leadership.
They needed his ability to galvanise a group, just as he did after England’s excruciating World Cup campaign.
But what they got was a disappearing act that is now likely to go on for weeks and possibly months, depending on what happens at the disciplinary hearing.
When a club has taken as many blows as Saints have this season, it needs its strongest characters to lead everyone out of the darkness.
But with Hartley having been dismissed, skipper Tom Wood was replaced and the light at the end of the tunnel was lost.
Wood was probably the best Saints performer and that remained the case after the game, when he spent a huge amount of time fronting up to the media.
He eventually headed off to get showered and changed a full hour after the match had finished.
It is not the first time he has emerged and given a lengthy account on what he has just been part of.
In fact, after every big defeat, he is the man who appears and tries to make sense of it all.
You could hear the pain in his voice.
It was the kind of pain etched on the face of the fans as they left the ground.
Everyone, from the chairman to the tea lady, is feeling it at the moment.
Saints needed a big performance to turn the tide.
They needed to swim rather than sink.
But sink they did.
And, barring a miraculous display in Dublin next Saturday night - similar to the one that occurred back in 2013 - their boat won’t be docking in Europe past January.
How they rated...
A tough night for the full-back, who had players running at him from all angles towards the end and it looked too much... 4
Came into the team in the absence of George North, but Saints found more space on the other wing... 4
Looked strong when Saints managed to get some front foot ball, but faded and missed a tackle for a Leinster try in the second period... 4
Started to get Saints moving early in the second half, with his impressive break and pass releasing Tuala, but he was forced off with an injury... 6
Didn’t get much of a chance to make impression as he was forced off with an injury after just 25 minutes... 4
Played with pace at the start of the second period, but was unable to exert any influence as Leinster got back in the ascendancy... 5
Looked reinvigorated after his time away, but his influence on the game faded as Leinster reclaimed control... 5
Was up against a tough Leinster scrum and Saints struggled in that area at times during an extremely difficult night... 4
Poured everything into the first half and came back on after Hartley’s dismissal but probably wished he didn’t have to as Leinster rubbed salt in the wounds... 5
The prop started really strongly and he played his part in defence, but Leinster took Saints on up front... 5
Missed a tackle for Leinster’s first try, but bounced back and put himself about... 5
Tried to bring his England form back to the Gardens and did put a shift in, but the Irish pack were too hot to handle in the end... 5
Kept his place in the team ahead of Teimana Harrison after a string of decent showings, but Leinster stopped him having a big influence... 4
The stand-out Saints player, the skipper threw himself about with relentless abandon and was unfortunate to be sacrificed... 6
A huge ask for him to play so many energy-sapping games in succession and Leinster stood up to him well... 5
Replacements (who played more than 20 minutes)
AHSEE TUALA (for Elliott 25)
Made a positive impression immediately as he landed a jackhammer hit on Isa Nacewa and proceeded to score Saints’ only try of the game... 6
API RATUNIYARAWA (for Paterson 57)
Came on at a difficult time in the game and it certainly didn’t get any easier as Leinster ran riot... 5
TEIMANA HARRISON (for Picamoles 57)
Has made too many appearances from the bench for his liking and this was certainly not an enjoyable one... 5
CHRON STAR MAN - Tadhg Furlong (Leinster)