A couple dies after an arsonist lights their house on fire and intentionally traps them inside. While the Five-0 taskforce investigates this heinous crime, Danny prepares to enjoy a weekend getaway with his girlfriend Amber.
When the team goes to the crime scene, they meet a gung-ho ATF agent from California, Kathy Millford. She has been chasing a serial arsonist who has been responsible for over two dozen fires over the last five years.
A suspicious man shows up at Kamekona's shrimp truck, looking for Amber. Kamekona reports the incident to Steve. Chin Ho discovers that the man is actually Amber's husband from New York, and Amber is really Melissa Simpson.
After a second fatal fire, McGarrett figures out that the firebug is targeting Agent Millford, taunting her with photographs. She had taunted the arsonist on television, and he lit her house on fire, killing her husband. She's was pulled from the case by the ATF. McGarrett gives in to her pleading to stay on the case.
Amber's ex tracks down her and Danny at their weekend getaway and knocks out Danny. He physically threatens her, but Danny recovers and attacks him. In the struggle, Danny is stabbed with a knife. Amber helps him into the car and hits her ex with the vehicle and runs him over on the way to the hospital.
The team identifies the fire-starter and after raiding his house realize that he has a bigger target in mind for his next fire: a hotel. They go to the hotel, but the arsonist captures Millford and nearly lights her on fire before being overpowered and arrested by McGarrett.