General Clive was in command of the British Force including about 300 members of the 39th Foot and he immediately took the offensive against the Surajah Dowlah, Nabob of Bengal, capturing Calcutta, Chandernagore and Cutwa.
The fifteen mile march to the battlefield on the 22nd June was made after torrential rain; and the troops had to wade waist-deep through the floods. Closing with the enemy at Plassey, near Calcutta, one of the 39th's company commanders, was Major Coote (later General Sir Eyre Coote, the victor of Wandewash in 1760 and Porto Novo in 1782) who supported Clive, with only one other officer, in voting for battle at the Council of War before Plassey.
Clive had only 3,000 men (over 2,000 being Indian Sepoys) and 15 light guns (but no cavalry) against the Nabob's army of over 60,000 infantry, cavalry, 50 heavy guns and some lighter ones (the latter manned by French gunners).
There was again a heavy downpour at noon on the 23rd, after which the enemy cavalry attacked but with the 39th having kept their powder dry, they, with desperate bravery, repulsed this attack. Following up, the 39th assaulted and captured the two foremost enemy positions. After some further resistance by the enemy, their main positions were carried and with the 39th's Grenadiers in the lead, the whole of Surajah Dowlah's force was routed from the field.
The formidable enemy force was completely defeated by the small British force, who captured the whole of the enemy's camp, baggage, guns and stores. This victory constituted one of the most complete and overwhelming achievements in military history. As a result of the victory, the whole of Bengal, a territory larger and more populous than Great Britain, came under British rule. In the battle the 39th bore the leading part, being in the centre of the line of attack, and so distinguished itself by its undaunted bravery and conspicuous valour that it was given the proud motto 'Primus in Indis' (First in India) and the battle honour 'Plassey' for its colours. Thus the motto bears a double meaning, for it not only records the fact that the Regiment was the first of the King's regiments to serve in India, but was first in bravery and courage in the battles of those days. The 39th was also prominent in other great actions in India, winning added fame in the battles against the combined French and native forces at Nellore, Trichinopoly, Wandewash and elsewhere.