When Monk stops by his local firehouse to have his smoke detectors inspected, a fire breaks out in a nearby home, calling away the fire company and leaving Monk alone with a retired fireman named Rusty. Soon thereafter, an unidentified man enters the firehouse. Rusty confronts the intruder, who hits Rusty over the head with a shovel. When Monk responds, the intruder throws solvent into Monk's eyes and makes his escape, leaving Rusty dead and Monk blind.
With doctors unable to set a timetable for a recovery, Monk must face the possibility that he will never see again. It's a devastating blow, but Natalie, Stottlemeyer, and Disher encourage Monk to keep his spirits up by becoming involved in the investigation of Rusty's murder. Monk goes to the scene of the crime, and sure enough, despite his inability to see, he discovers a clue. By feeling around, Monk can tell that a fireman's coat and helmet are missing.
The police find the missing coat in the possession of a drifter, who found it in a garbage dumpster. The garbage dumpster turns out to be located near the scene of a fire, the same fire which called away the fire company on the day of Rusty's murder. Monk suspects a connection between the fire and Rusty's killing.
Monk goes to the scene of the fire, which has been ruled an accident, to investigate. Monk, who's grown more comfortable being blind, feels his way around. He makes note of an imprint of a galloping horse etched in a coffee table, and after a little more snooping, comes up with a theory: the victim of the fire, a young woman named Stefanie Preston, was actually strangled. The killer then started what appeared to be an accidental fire, and left the building. The killer soon realized that he left something in the fire that could be traced back to him, so he needed a way back in. He went to the firehouse and stole a coat and hat, and disguised himself as a fireman to gain reentry to the fire scene, where he retrieved whatever he left behind.
Next, Monk and the gang go to a construction site where Stefanie Preston last worked as a temp. They talk to her boss, Peter Breen, who says he was working at the construction site when she died. When another construction worker, Eddie, walks over to remind Breen that he has his car keys, Monk hears a sound he heard the day Rusty was killed: the sound of squeaky shoes. Monk follows the sound. Unfortunately, nobody sees Monk leave the group, and Monk finds himself alone with Eddie. Monk tries to apprehend him, but Eddie gets away. By the time Natalie, Stottlemeyer and Disher catch up to Monk, Eddie turns up dead.
The killer is dead, but Monk is not satisfied that Eddie acted alone. What motive did he have to kill Stefanie Preston? And then Monk remembers something: Eddie said he has Peter Breen's keys. Breen drives a Ferrari, and Ferrari's key chain logo is a galloping horse, just like the imprint left in Stefanie Preston's coffee table! It was Peter Breen's keys that Eddie went back into the fire to retrieve! Eddie Murdoch killed Stefanie Preston, but Peter Breen, who was having an affair with her, paid him to do it.
The Captain and Monk head down to the morgue to retrieve the keys from Eddie's body. Just as they find the keys, Peter Breen suddenly appears, and he wants that incriminating key chain! He hits Stottlemeyer on the head, and then goes after Monk. Monk grabs Stottlemeyer's gun and, with his eyesight just beginning to return, shoots Breen, seriously wounding him just in the nick of time. Monk will soon make a full recovery, but this case shows that Monk can solve the murder and save the day, even when he can't see a thing.