Cardiff suffered a gut-wrenching defeat to English champions Harlequins in a 10-try Heineken Champions Cup thriller at Cardiff Arms Park.
The Blue and Blacks led by two points at half-time thanks to tries from Owen Lane and James Ratti.
Harlequins took the lead early in the second half thanks to Luke Northmore but Cardiff burst into life with three tries in little more than five minutes thanks Lane, Dillon Lewis and Corey Domachowski.
Danny Care breathed life into the English champions and they laid siege in the final 10 minutes, striking through half back duo Danny Care and Marcus Smith to bring scores level in the final 10 minutes.
They weren't left empty handed, picking up two bonus points as they prepare to visit defending champions Toulouse in the final pool stage match next week.
Harlequins dominated the opening exchanges at an empty Cardiff Arms Park after the Blue and Blacks failed to secure the kick-off. They went through a succession of phases and looked to make an early statement as they kicked a penalty to the corner.
Cardiff defended the set piece well and kept the Harlequins drive at bay but a number of phases later, and seemingly with no options available, Marcus Smith sent a pin-point cross-kick to the corner, which Lynagh collected to score.
Smith added the extras to give his side an early 7-0 advantage but Cardiff were undeterred and soon settled into the rhythm of the game.
They threatened on a number of occasions with Tomos Williams taking quick taps and Josh Adams, Rey Lee-Lo and Hallam Amos all making good yards.
The home side stretched the visitors from left to right and continued to ask questions before Dino Lamb snatched a key turnover after a tackle in the corner from Tyrone Green.
Cardiff were unfortunate to come away from their early venture into the Quins 22 without points but they did not have to wait long for the opening score.
It came from a counter attack launched by Amos, a phase later Shane Lewis-Hughes sliced through the defence thanks to a flat pass from Jarrod Evans.
The big flanker, making his first start in 11 months, could not keep his feet but possession was maintained and moments later Lane powered out of two tackles to burst clear.
Evans was on target to level the scores but as the 20 minute mark approached, the Blue and Blacks took the lead.
The platform was laid as Lee-Lo rolled back the years with a break from deep inside his half as he handed off Luke Northmore on the outside. He tour into the Harlequins half and all odds suggested a try with Adams in support but the visitors defence scrambled extremely well and the Lion was held short.
Now, playing with advantage, Cardiff hammered at the English champions and Evans attempted a cross kick which Green failed to cling onto but evaded on rushing Lane.
They came back and kicked to the corner and displayed plenty of patience as they went through the phases before Ratti powered over. Evans converted to give his side a 14-7 lead much to the delight of the hardy souls watching from the Westgate Street flats.
As the encounter progressed, both teams continued to throw caution to the wind and attacked from all opportunities.
One breathless period saw Tomos Williams make a remarkable break from the back of scrum, beat three defenders and throw a monstrous pass to Lane who almost broke clear.
Quins managed to snatch possession and went on the counter before the ball returned to Cardiff and a miss-hit, attacking kick from Seb Davies almost released Amos after a cruel bounce evaded Adams.
The Londoners also had their moments with a delicious line from Joe Marchant breaking the home defence before Lane saved the day with a fine tackle.
They kicked a succession of penalties to the corner but while their driving line out was held at bay, they turned the screw at scrum time. For five minutes they were camped on the Cardiff five-metre line with Dmitri Arhip sin-binned.
Dillon Lewis was called into action with Lane coming off but the pressure continued and playing with the advantage, Smith again sent a cross kick, which Caden Murley failed to collect but took Amos out in the process and Green pounced to score.
Smith failed to convert with the last kick of the half and Cardiff entered the changing rooms with a slender 14-12 lead.
Harlequins very almost scored virtually straight from the kick-off as Smith chipped over the Cardiff defence for Marchant. Adams made a fine try-saving tackle but the centre managed to find Danny Care with an offload.
It was a real sucker punch for the Blue and Blacks but as Smith lined up the conversion the TMO intervened and referee Tuai Trainini ruled the try out for offside from the kick and a forward pass.
Cardiff should have claimed a third try five minutes into the half as they worked an overlap on the right. Josh Adams drew the defence but Amos was unable to hang on.
With Young’s side still reduced to 14, Harlequins finally took advantage as Smith looked to break right but came back left and slipped a short pass to Alex Dombrandt.
The number eight burst through and threw a fine pass to fellow former Cardiff Met star Lujke Northmore to score.
Smith converted and the visitors now held a five point lead just as Cardiff returned to full compliment, but it did not last long.
After a penalty on midfield and dissent, which saw Harlequins marched back another 10, Cardiff kicked to the corner. They remained in the red zone for more than five minutes, a period which saw Dombrandt sent to the bin, and the pressure eventually told as Cardiff took a succession of quick taps and Lewis burrowed over.
Evans dissected the posts to give his side a two point lead and two minutes later the Blue and Blacks claimed a sensational score.
Quick hands saw Cardiff create space and make ground down the left, Amos kicked through and under huge pressure Green fumbled and Lane hacked a head and acrobatically finished with millimetres to spare.
Evans fired narrowly wide from the touch line but unbelievably Cardiff claimed a fifth try from the restart and this one was even better.
Lee-Lo again made a break on the outside arc and released Davies with a flick pass out the back. The second-row got in on the act with a basketball-style offload over the top to Lewis-Hughes, who quickly released Lane.
The wing now bursting with confidence looked to cut inside but stepped off his left in devastating fashion to beat Green on the outside. He deserved the try but Smith and Muir recovered to drag him down.
However, the damage was done and after a quick dart from Williams, Domachowski found himself in space and needed no second invite to pile drive between two defenders.
Evans converted to make it 33-19 but just as they seemed to edge away, Harlequins stormed back.
It appeared as though Ellis Jenkins had snatched a fine turnover, but not for the first time Trainini refused to reward the jackal and instead penalised the flanker. Harlequins kicked to the corner and Care wriggled over from a set play with Smith converting.
Frustratingly, while Cardiff were not being rewarded at the breakdown, Harlequins claimed two important turnover penalties in quick succession.
Harlequins laid siege to the Cardiff try line as the game entered final 10 minutes and Trainini continued to penalise the hosts, who were firmly under the cosh.
Myhill was sent to the bin on 72 minutes and Harlequins turned the screw at the scrum. The pressure eventually told as England international Smith skipped his way past the defence, dotting down to set-up a straight-forward conversion to bring things level.
However, a late penalty from Smith - the final kick of the game - saw the English champions snatching a last gasp winner at the Welsh capital, keeping their 100 per cent record in this year's competition alive.