He's worried about Elphaba, and instead of staying home he's out stalking her around town.
He follows her through Emerald City and she finally stops at a theater.
She's waiting for someone to come out.
Fiyero is nervous for her and watches her from a nearby library:
He didn't know if he was there to prevent what she was about to do, or to save whomever he could from the catastrophe, or to tend anyone hurt accidentally, or even maybe just to witness it, so he could know more about her. (3.15.9)
Fiyero is a bit put out that Elphaba doesn't sense that he's nearby. So much for epic romance and soul mate connections and whatnot. Perhaps Fiyero has been reading Twilight in his spare time.
He wonders what she's up to and looks for her colleagues.
Then he sees Madame Morrible exiting a carriage.
She's the target, which is kind of fitting really. Morrible's little career-advising session played a large role in setting Elphaba on this path to the Emerald City and the resistance.
Elphaba is getting ready to assassinate Madame Morrible when a bunch of little girls rush out from a nearby school and swarm around the woman.
Elphaba no longer has a clear shot and she can't do the deed.
Fiyero watches her collapse in shame and disappointment.
Then he loses sight of her and he rushes back to her apartment.