The synth paranoia has gripped Diamond City with a particular ferocity, ever since the Broken Mask Incident, and shows no signs of subsiding even as late as 2287. The Sole Survivor can get a taste of the situation when they enter Diamond City before locating Nick Valentine.
This "quest" is a scripted encounter in Diamond City, where the player runs into an altercation in the Diamond City market. Kyle has taken a gun to his own brother, Riley, convinced that he is a synth. Regardless of what the player does, Riley dies, shot to death by Diamond City security. The only exception is that the player can kill Riley to hasten the inevitable conclusion.