Five goals, three penalties - two missed - and a sending-off, this game had everything...except the proud performance and vital three points that Hammers fans so badly craved.
On an afternoon to forget for Alan Pardew's men, Marlon Harewood's 49th minute penalty was purely a consolation, as Christian Dailly's own goal, a double strike from Tim Cahill, and one from Nick Chadwick saw Millwall romp to victory.
To make matters worse, young goalkeeper Stephen Bywater was shown the first red card of his career for what was deemed to be a professional foul on Lions striker Neil Harris - leaving 10-man Hammers with a mountain to climb for the final half an hour of the match.
However, already 3-1 down at that stage, the visitors hadn't displayed any suggestion that a comeback might be on the cards, as Millwall enforced their superiority all over the field and simply showed more passion, desire and strength in every department.
Pardew had received a major boost prior to kick-off when Matt Etherington passed a late fitness test on the badly-bruised shin he sustained at Sunderland last week. That meant a slight reshuffle, as Jobi McAnuff made way and Nigel Reo-Coker moved out to the right, with Kevin Horlock keeping his place alongside Michael Carrick in the centre as the Hammers boss prepared for the expected battle in midfield.
After a fairly tense and even opening to the game on a blustery and difficult day in south east London, the hosts were gifted the opportunity to take the lead in the 17th minute, when Etherington bundled Paul Ifill over just inside the penalty area and the referee's assistant on the far side signalled for a foul.
Striker Neil Harris stepped up to take the spot-kick but Bywater came to the rescue as he dived low to his right to block the effort, and Horlock was on hand to clear the loose ball. It was the young goalkeeper's second penalty save since his promotion as David James' replacement in January, following his superb effort at Sheffield United in his first game back in the side, but sadly it turned out to be the only highlight of his afternoon today.
Hammers registered their only effort on target in the first half just moments later, when Horlock's speculative free-kick from 35 yards out clipped the top of the crossbar but, on 34 minutes, the hosts made amends for their penalty miss with the opening goal. Ifill collected in midfield before drifting out wide to the left and curling in a dangerous cross that Dailly stretched for but only succeeded in diverting into his own net.
The Hammers skipper and his defensive colleagues appeared to have been handed a bonus at the break when striker Danny Dichio - whose strength in the air had been a constant source of discomfort in the first half - was forced off with illness and replaced with young rookie Nick Chadwick.
However, within seconds of the restart, the Lions substitute had created a second goal for his team, as his deep cross from the left sailed past a static Horlock and was met by the unmarked Cahill, who headed past Bywater from six yards out.
That was the start of an incident-packed 15-minute spell, as Hammers hit back to give themselves a lifeline and then saw it snatched away again. The response to Millwall's second was immediate. In the 48th minute, Horlock's lofted free-kick to the far post was destined for the head of Bobby Zamora, only for Lions defender Matthew Lawrence to knock the ball past the striker with his forearm. Referee Jeff Winter had no hesitation in awarding the second spot-kick of the game, and Harewood slotted home from 12 yards.
Pardew then sent veteran striker Brian Deane on in an attempt to add more presence to the attack as Hammers pushed forward in search of an equaliser, but on 56 minutes the hosts restored their two-goal advantage. Tomas Repka was distracted by a slight altercation with Cahill after Millwall had been awarded a corner and, as the kick was floated in, the midfielder was again left unmarked to crash home an unstoppable left-footed volley.
Three minutes later, things went from bad to worse. A long ball over the top released Harris, who was caught by Bywater as he lofted the ball goalwards. At first, Winter appeared to have played the advantage but, after the ball had bounced over for a goal-kick, he made it a hat-trick of penalties by pointing to the spot again, and subsequently brandished the red card to leave Bywater distraught.
Etherington was sacrificed as reserve goalkeeper Pavel Srnicek came on for his Hammers debut, but the Czech's first job thankfully wasn't to pick the ball out of the net, as Cahill wasted the chance to grab a career-first hat-trick by blasting his spot-kick high and wide.
Attentions were then turned to matters off the pitch for a short while, as a small minority of Hammers fans allowed their frustrations to get the better of them, but a heavy police presence - including officers on horseback - thankfully diffused the situation and no further trouble was reported.
Sadly, on the pitch, things didn't get any better. With Millwall using their one-man advantage to clever effect and Hammers bereft of Etherington's counter-attacking danger, a miracle comeback never seemed likely.
Indeed, with 10 minutes remaining, the game was put firmly out of reach, as Millwall claimed a fourth that again owed more to shoddy defending from Hammers' point of view. Another long ball forward again caused confusion in the blustery conditions and Repka could only divert his clearance into the path of Chadwick, who gratefully blasted home from 10 yards out.
As the 3,000 or so stunned travelling supporters struggled to come to terms with a defeat that meant so much more than just three points in the race for promotion, Alan Pardew pulled no punches in his post-match summary.
"There are no excuses today, we've got to hold our hands up and admit that we were well beaten," said a grim-faced Hammers boss. "The players have let themselves down and let the fans down, and it just wasn't good enough.
"The back four were inept and, all over the pitch, we just didn't show the kind of fight and stomach that is required in a London derby against our nearest rivals.
"It really couldn't have been a worse day. Everything we planned for went against us and we just didn't deal with the dangers that Millwall possess. We didn't deal with them, we didn't deal with the conditions...we didn't deal with the whole scenario.
"We've come away from here with our tail between our legs and what is important now is that we give the right reaction. One thing is for sure, we are not as bad as today's performance would suggest, but it is up to the players to go out and show that next week."