The Prussian IV Corps (Bülow's) was the first to arrive in strength. Bülow's objective was Plancenoit, which the Prussians intended to use as a springboard into the rear of the French positions. Blücher intended to secure his right upon the Châteaux Frichermont using the Bois de Paris road. Blücher and Wellington had been exchanging communications since 10:00 and had agreed to this advance on Frichermont if Wellington's centre was under attack.General Bülow noted that the way to Plancenoit lay open and that the time was 16:30.

At about this time, the Prussian 15th Brigade was sent to link up with the Nassauers of Wellington's left flank in the Frichermont-La Haie area, with the brigade's horse artillery battery and additional brigade artillery deployed to its left in support. Napoleon sent Lobau's corps to stop the rest of Bülow's IV Corps proceeding to Plancenoit. The 15th Brigade threw Lobau's troops out of Frichermont with a determined bayonet charge, then proceeded up the Frichermont heights, battering French Chasseurs with 12-pounder artillery fire, and pushed on to Plancenoit. This sent Lobau's corps into retreat to the Plancenoit area, driving Lobau past the rear of the Armee Du Nord's right flank and directly threatening its only line of retreat. Hiller's 16th Brigade also pushed forward with six battalions against Plancenoit.

Napoleon had dispatched all eight battalions of the Young Guard to reinforce Lobau, who was now seriously pressed. The Young Guard counter-attacked and, after very hard fighting, secured Plancenoit, but were themselves counter-attacked and driven out. Napoleon sent two battalions of the Middle/Old Guard into Plancenoit and after ferocious bayonet fighting—they did not deign to fire their muskets—this force recaptured the village.