Andreas Voglsammer scored his first goal in English football to set Millwall on the road to a vital victory over Watford in their fight for a Championship play-off place.
The German striker's first-half opener was followed by Zian Flemming's 10th of the campaign after the Hornets were reduced to 10 men by Hassane Kamara's red card in the 38th minute.
That numerical disadvantage, and a woeful performance, inflicted only a second defeat in the past nine games on Slaven Bilic's Watford team, denting their own push for promotion to the Premier League.
Tom Bradshaw's good work created the opening for Voglsammer, a summer signing from Union Berlin, and in his 19th appearance for the south London club, he got on the scoresheet for the first time by rolling his shot into the corner.