Watford produced an outstanding display to secure a lucrative FA Cup third round replay at Chelsea.
First Division Watford twice took the lead - through Heidar Helguson's controversial header and a Gavin Mahon goal.
But they were pegged back by an Eidur Gudjohnsen penalty and Frank Lampard's deflected shot.
The major talking point from the game was Helguson's opener - which struck the crossbar and bounced downwards on to the goalline but was adjudged to have gone in.
The referee's assistant, Dave Bryan, deemed the ball had crossed the line despite television replays showing it had not.
Chelsea goalkeeper Neil Sullivan, who endured a torrid first half, remonstrated with the officials but his protests fell on deaf ears.
The goal was no less than Watford deserved for their tenacity and refusal to be overawed by their more illustrious opponents.
VICARAGE ROAD DRAMA
5 mins: Helguson heads Watford into the lead with a debatable goal
33 mins: Gudjohnsen scores from the penalty spot
34 mins: Mahon puts Watford back into the lead
41 mins: Lampard equalises courtesy of a deflection off Hyde
Helguson proved a constant threat to Chelsea's hesitant defence, Ardley led by example, while the trickery of Paul Devlin on the right frequently left Celestine Babayaro bamboozled.
The home side sensed Chelsea's unease and Micah Hyde and Paulo Vernazza were unlucky not to extend the Hornets' advantage.
In truth, Chelsea offered very little, their play disjointed and passing wayward.
Marcel Desailly had an excellent opportunity to restore parity soon after Watford had scored, but the French international blazed over from three yards out.
Chelsea did level matters on the half hour when Jesper Gronkjaer tumbled at the feet of goalkeeper Lenny Pidgeley - on loan from Stamford Bridge - after being sent through on goal by Lampard's pass.
Gudjohnsen coolly slotted home the resulting spot-kick.
Gudjohnsen and Lampard were on target for Chelsea
Instead of building on their equaliser, Chelsea relinquished the lead straight away.
Watford once more exploited the visitors' defensive disarray as Marcus Gayle headed back across goal for Mahon to nod home Watford's second.
There was to be another twist before half-time. Gronkjaer was given time and space on the right and picked out the onrushing Lampard, whose deflected shot sneaked past Pidgeley.
Although the second half failed to live up to the excitement of the first, both teams had chances to win the game.
Watford may feel aggrieved that they were not awarded a penalty when goalscorer Mahon was brought down in the area by William Gallas.
But Ray Lewington's men will go into the replay with their heads held high after an impressive performance against Chelsea's expensively-assembled side.
Watford: Pidgeley, Ardley, Dyche, Gayle, Smith, Devlin, Hyde,
Mahon, Vernazza, Cook (Webber 80), Helguson.
Subs Not Used: Chamberlain, Dyer, Doyley, Fitzgerald.
Chelsea: Sullivan, Johnson, Desailly, Gallas, Babayaro,
Gronkjaer, Makelele, Lampard, Geremi, Gudjohnsen, Mutu.
Subs Not Used: Hasselbaink, Cole, Melchiot, Huth, Ambrosio.
Referee: A Wiley (Staffordshire).