From The Anarchy: The East India Company, Corporate Violence, and the Pillage of an Empire, by William Dalrymple (Bloomsbury, 2019), Kindle pp. 161-162:
The bankers and merchants of Bengal who sustained Siraj ud-Daula’s regime had finally turned against him and united with the disaffected parts of his own military; now they sought to bring in the mercenary troops of the East India Company to help depose him. This was something quite new in Indian history: a group of Indian financiers plotting with an international trading corporation to use its own private security force to overthrow a regime they saw threatening the income they earned from trade. This was not part of any imperial masterplan. In fact, the EIC men on the ground were ignoring their strict instructions from London, which were only to repulse French attacks and avoid potentially ruinous wars with their Mughal hosts. But seeing opportunities for personal enrichment as well as political and economic gain for the Company, they dressed up the conspiracy in colours that they knew would appeal to their masters and presented the coup as if it were primarily aimed at excluding the French from Bengal for ever.
By 1 May, a Secret Committee made up of senior Company officials in Bengal formally resolved to join the conspiracy: ‘The Committee were unanimously of the opinion that there could be no dependence on this Nabob’s word, honour and friendship, and that a revolution in the Government would be extremely for the advantage of the Company’s affairs.’
The Secret Committee then began to haggle over their terms of service, again using Khwaja Petrus as the intermediary for their coded correspondence. Before long, Mir Jafar and the Jagat Seths had significantly raised their offer, and were now promising the participants Rs28 million, or £3 million sterling – the entire annual revenue of Bengal – for their help overthrowing Siraj, and a further Rs110,000 a month to pay for Company troops. In addition, the EIC was to get zamindari – landholding – rights near Calcutta, a mint in the town and confirmation of duty-free trade. By 19 May, in addition to this offer, Mir Jafar conceded to pay the EIC a further enormous sum – £1 million – as compensation for the loss of Calcutta and another half a million as compensation to its European inhabitants.
On 4 June a final deal was agreed. That evening, Khwaja Petrus obtained for Watts a covered harem palanquin ‘such as the Moor women are carryed in, which is inviolable, for without previous knowledge of the deceit no one dare look into it’. Within this, the Englishman was carried into Mir Jafar’s house to get the signatures of the old general and his son Miran, and to take their formal oath on the Quran to fulfil their part of the treaty obligations. On 11 June, the signed document was back in Calcutta with the Select Committee, who then countersigned it. The next evening, pretending to set off on a hunting expedition, Watts and his men decamped from Kasimbazar and made their escape through the night, down the road to Chandernagar.
On 13 June 1757, a year to the day since Siraj had begun his attack on Calcutta, Clive sent an ultimatum to Siraj ud-Daula accusing him of breaking the terms of the Treaty of Alinagar. That same day, with a small army of 800 Europeans, 2,200 south Indian sepoys and only eight cannon, he began the historic march towards Plassey.