Nine-man Millwall were thrashed by a rampant Birmingham side at the New Den this afternoon.
Goals from Curtis Davies, Marlon King (2), Chris Burke, Adam Rooney and Nathan Redmond ensured that the Midlands side climbed into the play-off positions.
The home side had a glorious chance after just three minutes when Hameur Bouazza found Danny N'Guessan in the box, only for the Millwall man to somehow head wide.
The Lions continued to look the better side when Beausejour was forced to clear off his own line following a volleyed effort from Harry Kane.
Kane then came close again, heading just wide of Boaz Myhill's far post after another Bouazza cross.
Despite this pressure, however, it was the visitors who opened the scoring with their first attack of the match. A free-kick was lofted in by Jordon Mutch, allowing Curtis Davies to power the Blues into an 18th-minute lead.
Millwall's misery was compounded just three minutes later when Shane Lowry was shown a red card following a tackle on Nikola Zigic, with the referee appearing to take guidance from his linesman before issuing the marching orders.
The quickfire goal and dismissal seemed to take some of the wind out of Millwall's sails, but Kane continued to threaten, fizzing a shot just past Myhill's post in an effort to level the sides.
Only a last-gasp block by Liam Ridgewell towards the end of the half prevented Kane from getting that equaliser as he looked to be in on goal, only for the ex-Aston Villa defender to thwart his effort.
It was Myhill who was alert to deny Kane minutes later, tipping the lively striker's shot over the crossbar.
Birmingham began the second half looking stronger than their opponents, with Davies only being denied a second goal of the game thanks to a fine save by David Forde.
King then continued the pressure with a bending free-kick that flew just wide. Millwall would have thought they were still in the game, however, until they were struck by a telling blow just before the hour mark.
Alan Dunne became the second man sent-off for the Lions for a dangerous tackle on Zigic and, just two minutes later, King doubled Birmingham's lead with a stylish turn and finish.
King came close again when Forde tipped his effort around the post before setting up Burke to make it 3-0 to the visitors.
The home side thought that they had a consolation goal after Liam Feeney stabbed home Bouzza's free-kick, but the linesman ruled it out for offside.
Just minutes later, the away side extended their lead even further as Burke teed up Adam Rooney who thumped his shot in from the edge of the box.
King then made it three goals in 10 minutes for the Blues when he rounded Forde before slipping the ball into an empty net.
The scoring was finally finished off by teenage substitute Nathan Redmond in the 92nd minute as Birmingham recorded an emphatic away win.