After nine defeats and a draw with Castres, Harlequins' season finally exploded into life as they raced into a remarkable 33-0 lead by half time, securing the bonus point for four tries after only 14 minutes.
Harlequins opened their account after just 13 seconds through Mel Deane and worked six tries in all, with blindside flanker Nick Easter scoring his first two for the club.
Fullback Tom Williams, England prop Mike Worsley and lock Roy Winters all got in on the act as Quins lifted themselves above Worcester and off the foot of the Zurich Premiership table.
They outplayed Saracens in every department and only conceded late tries to Taine Randell and Paul Bailey when the game was long secured.
The victory finally brought to a close Harlequins' club record run of nine successive league defeats, dating back to the final game of last season.
After losing to Worcester, Harlequins had been steadily improving but reaped little reward from positive displays against Gloucester and Leicester in the league, and Munster and Castres in the Heineken Cup.
But tonight they earned their just reward for a blitzkrieg of a performance – and it was on the cards right from the kick-off.
Bizarrely, a week after conceding the fastest try ever against Wasps, Harlequins pounced on a mix-up between Jackson and centre Dan Harris to take the lead after just 13 seconds.
Harris dropped the attempted switch pass and Deane, only starting because Will Greenwood was away on England duty, scooped up the loose ball and sauntered unchallenged over the line.
The Harlequins faithful at The Stoop, who had endured one misery after the next so far this season, could hardly believe what they were witnessing when Easter barged over to score his first try for the club five minutes later.
Saracens could do nothing to relieve the pressure and managed just the one foray into the Harlequins 22 before they were shredded again.
Keogh, the Irish winger and a constant threat down the left, burst onto a mispass from fly-half Jeremy Staunton, jinked through and sent Williams over for the third try inside nine minutes.
By now Saracens had lost all semblance of organisation and they then suffered the ignominy of being undone by a neat exchange of passes down the wing – by two props.
Ceri Jones timed a lovely little miss-pass to Worsley outside him and his front row colleague brushed aside both Harris and Scottish fullback Robbie Kydd to score in the corner.
Staunton missed his first conversion of the evening but Harlequins already led 26-0. The win and the bonus point were already secure and the game was less than a quarter of an hour old.
Still Harlequins poured forward, stung perhaps by memories of the Castres game when they let a healthy lead slip, and exploited a rag-tag Saracens outfit who could hardly escape their own 22 as fly-half Glen Jackson, the New Zealand Maori recruited from Waikato, made a shocking first start.
After Easter had barged over for his second try of the evening, and Harlequins' fifth, Saracens owner Nigel Wray left his seat in the West Stand.
Which might have been just as well for Jackson, who soon after dropped out from his 22 straight to Keogh. His blushes, on that occasion at least, were saved by flanker David Seymour who forced Keogh to knock on as he attempted to ground the ball.
But Harlequins continued in similar vein after the interval, forcing Saracens deeper and deeper before Winters charged down an attempted clearance from scrum-half Moses Rauluni and touched down.
Staunton's fifth successful conversion from six attempts opened Harlequins a 40-0 lead after just 45 minutes.
Jackson was immediately replaced by Australian Mark Bartholomeusz and Canadian international Morgan Williams – who could have been representing his country against England over the road at Twickenham tomorrow – was introduced for Rauluni.
Eventually Saracens crossed the line, to a chorus of ironic cheers, after 53 minutes when captain Randell finished off a well-worked move in the corner.
Bailey added a second for Saracens but no-one at The Stoop barely noticed as they celebrated a long-awaited change in fortune.
For Harlequins:Tries: Deane, Williams, Worsley, Easter 2, WintersCons: Staunton 5
For Saracens:Tries: Randell, Bailey