You seem to be referring to Macbeth's plan to have Banquo and his son Fleance murdered. This business is thoroughly covered in Act 3, Scene 1, from the very beginning; but the most specific description of his plan occurs in lines 147-163 of Act 3, Scene 1, beginning with Macbeth's words, "Your spirits shine through you." The part of his plan that is not carried out is the murder of Fleance. In these lines Macbeth tells the murderers: "And with him / (To leave no rubs nor botches in the work) / Fleance, his son, that keeps him company, / Whose absence is no less material to me / Than is his father's, must embrace the fate / Of that dark hour." In Act 3, Scene 3, Fleance escapes because it is a dark night and the torch accidentally gets extinguished. Therefore Macbeth's plan is thwarted, and Banquo's son survives to fulfill the Witches prophecy that Banquo will be father to a whole line of kings.