The Athenians, after plans had been arranged between themselves and their correspondents both as to words and actions, sailed by night to Minoa, the island off Megara, with six hundred heavy infantry under the command of Hippocrates, and took post in a quarry not far off, out of which bricks used to be taken for the walls; while Demosthenes, the other commander, with a detachment of Plataean light troops and another of Peripoli, placed himself in ambush in the precinct of Enyalius, which was still nearer. No one knew of it, except those whose business it was to know that night. A little before daybreak, the traitors in Megara began to act. Every night for a long time back, under pretence of marauding, in order to have a means of opening the gates, they had been used, with the consent of the officer in command, to carry by night a sculling boat upon a cart along the ditch to the sea, and so to sail out, bringing it back again before day upon the cart, and taking it within the wall through the gates, in order, as they pretended, to baffle the Athenian blockade at Minoa, there being no boat to be seen in the harbour. On the present occasion the cart was already at the gates, which had been opened in the usual way for the boat, when the Athenians, with whom this had been concerted, saw it, and ran at the top of their speed from the ambush in order to reach the gates before they were shut again, and while the cart was still there to prevent their being closed; their Megarian accomplices at the same moment killing the guard at the gates. The first to run in was Demosthenes with his Plataeans and Peripoli, just where the trophy now stands; and he was no sooner within the gates than the Plataeans engaged and defeated the nearest party of Peloponnesians who had taken the alarm and come to the rescue, and secured the gates for the approaching Athenian heavy infantry.
After this, each of the Athenians as fast as they entered went against the wall. A few of the Peloponnesian garrison stood their ground at first, and tried to repel the assault, and some of them were killed; but the main body took fright and fled; the night attack and the sight of the Megarian traitors in arms against them making them think that all Megara had gone over to the enemy. It so happened also that the Athenian herald of his own idea called out and invited any of the Megarians that wished, to join the Athenian ranks; and this was no sooner heard by the garrison than they gave way, and, convinced that they were the victims of a concerted attack, took refuge in Nisaea. By daybreak, the walls being now taken and the Megarians in the city in great agitation, the persons who had negotiated with the Athenians, supported by the rest of the popular party which was privy to the plot, said that they ought to open the gates and march out to battle. It had been concerted between them that the Athenians should rush in, the moment that the gates were opened, while the conspirators were to be distinguished from the rest by being anointed with oil, and so to avoid being hurt. They could open the gates with more security, as four thousand Athenian heavy infantry from Eleusis, and six hundred horse, had marched all night, according to agreement, and were now close at hand. The conspirators were all ready anointed and at their posts by the gates, when one of their accomplices denounced the plot to the opposite party, who gathered together and came in a body, and roundly said that they must not march out- a thing they had never yet ventured on even when in greater force than at present- or wantonly compromise the safety of the town, and that if what they said was not attended to, the battle would have to be fought in Megara. For the rest, they gave no signs of their knowledge of the intrigue, but stoutly maintained that their advice was the best, and meanwhile kept close by and watched the gates, making it impossible for the conspirators to effect their purpose.
The Athenian generals seeing that some obstacle had arisen, and that the capture of the town by force was no longer practicable, at once proceeded to invest Nisaea, thinking that, if they could take it before relief arrived, the surrender of Megara would soon follow.
Iron, stone-masons, and everything else required quickly coming up from Athens, the Athenians started from the wall which they occupied, and from this point built a cross wall looking towards Megara down to the sea on either side of Nisaea; the ditch and the walls being divided among the army, stones and bricks taken from the suburb, and the fruit-trees and timber cut down to make a palisade wherever this seemed necessary; the houses also in the suburb with the addition of battlements sometimes entering into the fortification. The whole of this day the work continued, and by the afternoon of the next the wall was all but completed, when the garrison in Nisaea, alarmed by the absolute want of provisions, which they used to take in for the day from the upper town, not anticipating any speedy relief from the Peloponnesians, and supposing Megara to be hostile, capitulated to the Athenians on condition that they should give up their arms, and should each be ransomed for a stipulated sum; their Lacedaemonian commander, and any others of his countrymen in the place, being left to the discretion of the Athenians. On these conditions they surrendered and came out, and the Athenians broke down the long walls at their point of junction with Megara, took possession of Nisaea, and went on with their other preparations.