Manchester United looked like Premiership leaders, while bottom of the table Watford resembled a team waiting for relegation to be confirmed.
Watford came to frustrate the Red Devils and managed it for 20 minutes until Cristiano Ronaldo broke the deadlock from the penalty spot.
But Watford's hopes of achieving a damage limitation exercise evaporated as three goals in 12 minutes from the hour onwards condemned them to this heavy defeat.
Ronaldo, rested for Saturday's 2-1 fourth round FA Cup win over Portsmouth, returned to lead Watford a merry dance.
One audacious piece of skill in the 24th minute summed up his night.
Faced by Jordan Stewart and Hameur Bouazza on the right, he left them dizzy with a series of step-overs and then put the ball through Stewart's legs to set up a chance for Gary Neville.
Ronaldo was the epitome of coolness in the 20th minute when he opened the scoring from the spot.
Michael Carrick played the ball to Wayne Rooney in the Watford box. As Rooney seemed set to find Ole Gunnar Solskjaer, the Norwegian had his legs taken from under him by Jay DeMerit.
Referee Mike Dean awarded the penalty and Ronaldo put the ball on the spot, stepped up, shimmied and then drove the ball right-footed into the bottom right corner of the net as he sent Hornets goalkeeper Richard Lee the wrong way.
United had to wait another 40 minutes before breaching the Watford defence again.
Carrick worked the ball to Rooney and he found Kieran Richardson free on the left side of the Watford penalty area.
Richardson crossed towards Ronaldo at the near post, but Watford defender Lloyd Doyley beat him to the ball only to see his header loop beyond Lee and into his own net.
Henrik Larsson replaced the disappointing Solskjaer and the Swede was on target three minutes later.
Rooney played a superb one-two with Larsson as they cut through the middle of the Watford defence and it was Larsson who finished the move when he stroked the ball right-footed beyond Lee and into the net from 12 yards.
United's fourth goal three minutes later was the pick of the bunch.
Ronaldo's measured chip landed perfectly for Rooney 12 yards out and, with Lee almost mesmerised, the England man scored with a fantastic right-footed lob over him and into the roof of the net.
United could have had a fifth goal five minutes from the end when Ronaldo rattled the foot of a post with shot from 20 yards.
In reality, this could have ended up a cricket score.
Richardson missed two good chances when free in the Watford box and left-back Gabriel Heinze should have done better than hit the ball straight at Lee when clear inside the penalty area.
Lee did make one marvellous save in the 68th minute when he kept out Ronaldo's point-blank header from a Rooney cross.
Watford's only real chance came 12 minutes from time when United goalkeeper Tomasz Kuszczak was forced to take Bouazza's 20-yard drive at full stretch.