It is well-known that fire bugs often work for fire brigades.
So it just happens that when someone in the brigade gets caught out as an arsonist, the fire chief (who happens to be the corrupt mayor’s brother), wants to take out someone.
All of this is interchangeable… the key aspect is that a small fire brigade team (can be urban or rural, voluntary or professional), embark on a side gig where they arson a place (including murder) and are the first on the scene (destroy evidence, empty the safe, whatever).
They are smart, only one job a year or so, so it gets lost in all the real jobs.
Someone in the brigade can be heroic (demonstrable, save a cat), and have a conscience.
A crime analyst (maybe even throw in AI) spots a pattern. An above the odds ratio of fires attended what involve homicide or theft. The superior says it is just random, that some place must have unusual activity, that is how stats work – outliers.
So the analyst starts investigating privately.
Obviously the finale is a fire. Heroism, cowardice, lives saved, bad guy burns, justice.
To make it special, that fire should be somewhere extra emotional. A school, an old people’s home, a sports stadium during a game (trying to stop stampedes, wrongly locked exit doors…), The more plausible and unexpected the better. Maybe a 35th storey rotating restaurant, theme park or zoo.
A main character has a family member in the 3rd act fire.
Actually, it could take on aspects of 70s disaster movies.
No love interests!