Fighting is already underway as the scene opens and Edward frantically questions Spencer Junior as to Gaveston's whereabouts. Just then, however, he catches sight of Gaveston, and the two men agree to flee in different directions. Edward accordingly says goodbye to both Gaveston and Lady Margaret, but only bids farewell to Isabella “for Mortimer, [her] lover's sake.” Everyone then leaves except for Isabella, who reiterates that she loves no one but Edward and wishes he would take pity on her.
It's difficult to pinpoint the exact moment at which Isabella definitively turns against her husband; alone on stage, she is still at this point professing her love for Edward. However, her loyalty does begin to falter later in this scene, and Edward's words to her here help explain why: Edward continues to harass her about her alleged infidelity regardless of what she says or does. Of course, Edward's words also speak unfavorably to his priorities, since he finds time even in a crisis to taunt his wife.
Lancaster, Warwick, and Mortimer Junior burst in. They are searching for Edward, and Mortimer interrupts Isabella's lament about her efforts to win her husband's affections to ask her where the King is. Isabella appears suspicious, so Lancaster clarifies that they have no intention of harming Edward, but simply want to “rid the realm of Gaveston.” Isabella therefore tells them that Gaveston has gone to Scarborough, unaccompanied by the King. Mortimer questions her on this last point, and Isabella explains that Edward hoped to force the nobles' army to split into smaller groups that could be more easily defeated.
Mortimer here characteristically favors action over words, brusquely cutting off Isabella's complaints about Gaveston to demand information. The fact that Isabella complies is one of the first hints that her allegiance may be shifting. Although she does make sure to clarify the nobles' intentions toward her husband before responding, Isabella not only points the nobles toward Gaveston but also divulges Edward's plans to them. The fact that it apparently did not occur to Edward that Isabella might betray him in this way reveals his lack of political acumen, but also points to the broader tendency of characters to underestimate the dangers posed by internal threats.
Heeding Isabella's words, the nobles decide to pursue Gaveston as a group. Before they leave, however, a dispute arises about where Isabella should go: Mortimer urges her to either stay in place or go with him, but Isabella declines, saying that Edward already suspects her of adultery. Once the nobles have left, however, Isabella's loyalty to her husband begins to waver, and she says she could “live with [Mortimer] forever.” She decides, however, to plead with her husband one last time and—if that fails—to go to France and appeal to her brother, the King of France, for help. She hopes, however, that Gaveston's death will make the trip unnecessary.
As Isabella's love for her husband begins to falter, she gives her first indication yet that she might consider Mortimer a replacement. This is not a decision to make lightly, however, since an affair on Isabella's part would be nearly as illicit and "threatening" as Edward's relationship with Gaveston. More specifically, it would open up the possibility that an illegitimate child might be passed off as Edward's, endangering the entire line of succession.