The movements of Lord Monteagle warranted the belief that he had received a warning of some kind that there was danger in town, for he removed from his house in London to one which he had at Hoxton, and on the 26th of October, six days before the proposed opening of Parliament, he, much to the surprise of his own family, ordered a good supper to be prepared there. Monteagle had formerly been engaged in the Spanish treason, and had written to Baynham, who was the emissary at Rome, and therefore was probably aware of some plot in agitation, but he had latterly obtained the confidence of the king and was one of the commissioners for the late prorogation.

As he sat at table about seven o'clock in the evening, a page handed to him a letter, which he said he had received from a tall man whose features he could not recognise in the dark. Monteagle opened the letter and seeing that it had neither date nor signature, he handed it to Thomas Ward, a gentleman of his establishment, to read aloud. It was as follows:—"My lord out of the love i beare to some of youer frends i have a caer of your preservacion therefor i would advyse yowe as yowe tender youer lyfe to devyse some exscuse to shift of youer attendance at this parleament for god and man hath concurred to punishe the wickednes of this tyme and think not slightlye of this advertisement but retyere to youre self into youre contri wheare you may expect the event in safeti for thowghe theare be no apparance of anni stir yet i saye they shall receyve a terrible blowe this parleament and yet they shall not seie who hurts them this cowncel is not to be contemned because it may do yowe good and can do yowe no harme for the danger is passed as soon as yowe have burnt the letter and i hope God will give yowe the grace to mak good use of it to whose holy protecion i comend yowe."

The astonishment of the guests at the hearing of this letter may be imagined. Lord Monteagle immediately hastened to town, and laid the letter before Cecil and some of the other ministers, the king being away still at Royston, hunting. Cecil determined that nothing should be done till the king's return. The next morning Ward, who had read the letter publicly at the supper-table, communicated the circumstance to Thomas Winter, and said the letter was in the possession of Cecil. Winter was thunderstruck, but put the best face upon the matter that he could, and pretended to laugh at the whole affair as a hoax on the credulity of Lord Monteagle; but no sooner was Ward gone than he flew to White Webbs and imparted the news to Catesby. Catesby at once attributed the letter to Tresham, and the more so as he had absented himself for several days on the pretence of having business in Northamptonshire. The question was whether he had revealed the particulars of the plot and the names of the conspirators. To ascertain the extent of the mischief and of the guilt of Tresham, he sent him an imperative message to come to White Webbs. Tresham obeyed the summons on the 30th of October, and met Catesby and Winter at this lonely house in Enfield Chase. They had made up their minds if they found him guilty, to shoot him on the spot. They charged him point blank with the discovery of the plot, and kept a searching gaze upon his countenance as he received their declaration. Had he faltered or shown any confusion, his doom would have been instant. But he exhibited the utmost calmness and firmness of expression, protesting most solemnly that he was innocent of the charge.

That Tresham was the writer of the letter, and that he had entered into a confidential understanding with Monteagle for the defeat of the plot, there appears every reason to conclude. His own avowal on the examination that such was his intention is borne out by all the examinations. The delivery of the letter whilst Monteagle was at supper with his friends, if it was done by Tresham, shows an intention that it should thus be made irrevocably public. The instant communication of Lord Monteagle's servant with Winter the conspirator, in order to warn them, confirms the idea that all this was planned between Tresham and Monteagle; but there is no reason to believe that Tresham had betrayed the names of his accomplices.

Catesby and Winter returned with Tresham to town, and Guy Fawkes was despatched to the cellar under the Parliament House to discover whether all was right there. Not a thing or a secret mark was disturbed. They then first told him why they had sent him, on which Fawkes complained of their distrust of his courage, and said he would visit the cellar every day till the 5th of November. Had Cecil not been still more cunning than the conspirators, had he made a stir and an inquisition, the aim of Tresham would have been effected, the conspirators would have escaped, and the plot have been put an end to without any catastrophe. But the artifice of Cecil lulled their suspicions and lured them on to their doom.

On the 31st of October James returned to town, and the letter was laid before him, with the particulars of its delivery. The king was struck by the account, read the letter several times over, and discussed the matter for two hours with his ministers. He boasted to Parliament on its opening, that it was his own bright suggestion that the receiving of the letter sent to Lord Monteagle implied that they were all to be blown up, and that he in consequence ordered the search of the cellars under the Parliament House. But this was a piece of consummate flattery on the part of his ministers, to make it appear the result of his superior sagacity; for we have direct evidence in the circular of the Earl of Salisbury, that the ministers were in possession of the secret, but he observes, "we all thought fit to forbear to impart it to the king until some three or four days before the sessions." In fact, the intelligence that the letter was in the hands of the king, and that the Council was consulting on it, was immediately conveyed to Winter by Monteagle's servant. Upon this Winter waited on Tresham at his house in Lincoln's Inn Walks, where Tresham, in great agitation, assured him that the existence of the mine was known to the ministers; that he knew certainly, but denied any knowledge of the person by whom the discovery had been made. He declared that they were all lost if they did not escape at once. From the moment the affair was known, Tresham had avoided further intercourse with the conspirators, meaning to appear ignorant of their concerns, for which reason he went about openly, and even offered his services to the Council.

The conspirators met to decide on their plan of action. Some of them advised instant flight to the Continent; Catesby, Winter, and others were perfectly convinced that Tresham was in communication with Monteagle, and perhaps with Cecil; but some of them would not believe such treason, and the arguments of Percy finally nailed them to their fate. This discussion took place on the 3rd of November. Percy conjured them to wait and see what the next day would bring forth, the very last day before the grand crisis. He represented all the labour, the anxieties, the plannings they had gone through, the costs they had incurred, the difficulties they had overcome, and he demanded whether, on the very point of complete success, they were to abandon their enterprise through the fears of a recreant colleague, who probably described what only his affrighted fancy pictured to him. He reminded them that his vessel still lay in the Thames at their service, and on the first positive proof of danger, they had only to hasten on board and drop down the river out of reach of their enemies.

These arguments prevailed, but they changed their plan of operations. Fawkes was still to keep guard in the cellar, Percy and Winter to superintend the necessary operations in London; but Catesby and John Wright were to hasten to Dunchurch and put Sir Everard Digby and the party on their guard.

On the evening of Monday the 4th of November, the Earl of Suffolk, in prosecution of his duty as Lord Chamberlain, to see all necessary preparations made for the opening of Parliament, went down to the House accompanied by Lord Monteagle.

After they had been some time in the Parliament chamber, on pretence that some necessary articles were missing, they went down to the cellars to make a search. They entered the vault where the mine was prepared and where Fawkes was at his post. The Lord Chamberlain, casually casting his eyes round the place, inquired by whom it was occupied, and who Fawkes was. The staunch traitor replied that it was occupied by Mr. Percy, whose servant he was; on which Suffolk observed in a careless manner, "Your master has laid in a good stock of fuel;" and he and Monteagle left the cellar. No sooner were they off the ground, than Fawkes hastened to inform Percy of what had occurred, but the warning was lost upon him. He persuaded himself that all was yet undiscovered, and Fawkes returned to the cellar to await the fatal hour.