Arson Inspector Morgan Conrad has come to the police for help. A building burned down under what she believes is suspicious circumstances but, since the fire superficially appears to be accidental, she is getting no support from her department. Shawn and Gus, who used to want to be firemen when they grew up, volunteer their assistance. Shawn, Gus, Lassiter and Juliet go with Morgan to the scene where they run into her boss, Army Johnson, who tells them that the building had no fire insurance, so there is no motive for arson. Plus, arson is so rare, he hasn't seen a case since way back to his firefighting days. But Morgan counters that one of the fireman on the scene reported smelling something sweet, a sign of an accelerant. As Shawn looks around the scene, he notices that one of the walls lacks insulation. He grabs a pipe and smashes it into the wall, revealing a blackened skeleton hidden inside. Now it's not just arson, it's murder.
At the coroner's office, they find out the victim was shot in the back execution style before he was burned. According to the report, Morgan says, trace amounts of the accelerant were found on the body. It is common and can be found in any hardware store in the city. While Morgan tracks the arson angle, the cops focus on trying to identify the victim. Shawn and Gus follow Morgan, but she refuses their assistance. Not to be deterred, they go to the fire station to investigate on their own, where they meet Fire Chief Dan, who tells them that their childhood hero, Fire Chief Paul and another firefighter, died in the Renkel fires of '98. While sad to hear that, the guys are there for a reason and tell Dan, who is wary of letting his men be questioned by a psychic, about the accelerant found at the scene. Dan is surprised to hear about that, but he is going off shift and gets his number two, Manetti, to assist them. As the guys gleefully participate in training drills with the firefighters, they hear that the fire was exceptionally easy to put out and that the building was completely empty of both people and furniture. Finally, as Gus is standing in for a CPR dummy, they learn that no firefighters missed their shift that night. Before they can follow up on that, the alarm sounds and the firehouse clears out. Shawn and Gus follow the crew to the fire in the hopes that the arsonist is still there. They are surprised to find Chief Dan there, as his shift was supposedly over. As he tells them he heard about the call over the scanner, Shawn sees flecks of pink paint on his shoulder. Shawn then drags Gus into the building where Gus smells the accelerant in the air and Shawn sees the ceiling is painted pink. Dan was in the building before the fire started, which now makes him their primary suspect.
Shawn meets Morgan and the cops at the coroner's office as another body was found in the latest fire. Shawn sees that this new victim had a gap in his teeth. They also find evidence that he was killed way back in '98. Shawn suspects that both bodies have been hidden in the buildings for ten years. Morgan confirms that both buildings were built that year and it would have been easy to kill and hide bodies at the construction sites. She also says that once again, this fire was easy to put out and the building had no insurance. Since the fires never pose too much risk to the firefighters, Shawn suspects one of their own is behind this and goes to find Gus at the station who has been tailing Chief Dan all day. Gus, nursing a bruise on his chest from playing CPR dummy, tells Shawn that Dan went to a hardware store, was on the phone all day, and asked about the accelerant. They follow Dan as he engages in some suspicious activity but it turns out he was just getting ready for a school field trip. As Dan throws them out, Shawn steals his cell phone. Five calls in a row to hardware stores all over the city. As Shawn and Gus relay their suspicions to Morgan, she gets a call about another fire - and another body. They meet Lassiter and Juliet at the new scene, where they are surprised to find not only another charred skeleton, but also the body of Chief Dan.
It turns out that Dan died of smoke inhalation and, as evidenced from a chest bruise matching the one Gus has, Shawn sees that someone tried to revive him. Lassiter says there were no traces of accelerant on him, ruling him out as the arsonist but not answering why he was there in the first place. Juliet says that their latest victim had a pacemaker and so they were able to identify him as Steve J. Renkel, as in the Renkel fires of '98. She pulls out mug shots of Renkel's accomplices and Shawn sees one of them has a gap in his teeth. All of the victims were members of Renkel's gang. It looks like they were hunted down ten years ago and now someone is trying to burn the evidence. There is one member of the gang who is still alive and, as the cops track him down, Shawn and Gus meet Morgan at Army's office to find more on the buildings themselves. They were all built a year after the Renkel fire and they are all scheduled for seismic retrofit. That's why they were all empty. The walls were going to be opened up for the retrofitting, which would have lead to the discovery of the bodies. Gus prints out the list of the other buildings scheduled for retrofit to find out where the arsonist will hit next. Morgan goes to check out the buildings as the cops bring in the surviving gang member. Bodie tells them of a time ten years ago when he was abducted to a construction site to be executed for the fires but the killer's gun jammed and he was able to escape. The only thing he remembers about the site was hearing hundreds of birds chirping. One of the buildings on the retrofit list is near a bird sanctuary. They call Morgan to alert her but she is already unconscious and being doused with accelerant by the arsonist. Shawn and Gus race to the building to find the arsonist is Army Johnson and, as they try to keep him talking so he won't light the fire, Shawn tells him he understands why he did it. He was a firefighter with the two guys who died in the Renkel fire. He was so devastated by their deaths that he sought revenge. Chief Dan figured it out and tried to stop him but was killed accidentally. Broken up by guilt, he sets himself on fire, but Shawn and Gus are able to get Army and Morgan out alive.