The Tangerines trailed 2-0 after only 21 minutes as Gianfranco Zola's side ran the show with goals from Ikechi Anya, after only four minutes, and Troy Deeney.
However, veteran Kevin Phillips threw the hosts a lifeline within seven minutes of the restart when he struck home from close range after good work from Chris Basham.
And the fightback was completed in the second minute of added time when Isaiah Osbourne latched onto a ball from the impressive Thomas Ince to strike home from 18 yards and ensure the points were shared.
Watford took the lead with their first real attack of the match when Matej Vydra put a goal on a plate for Anya and he duly despatched from only six yards out.
Michael Appleton's side were shocked into action but the disjointed nature of their play led to the visitors' second goal midway through the first half when Mark Yeates' cross was turned home by Deeney.
Appleton's side faced further problems after 28 minutes when Tiago Gomes entered the action for the injured Elliot Grandin.
After 49 minutes a header by Nathan Delfouneso brought a save from Watford goalkeeper Manuel Almunia. Within two minutes Basham had supplied Phillips with a goal and it was game on at last.
Put on the defensive at last, Watford looked vulnerable and Gomes went close from distance after 66 minutes. The busy Phillips was replaced by Wes Thomas with 23 minutes left as the Tangerines continued to probe for at least a point.
Gomes went close again before Ince hit the crossbar from 12 yards out.
With five minutes left, Thomas met a Stephen Crainey cross only to see his header brilliantly saved by Almunia.
It looked as if time had thwarted the Tangerines but two minutes into added time Osbourne turned home Ince's assist to make it 2-2.