An insert for a storm water treatment system includes an elongated baffle member that is configured to be mounted within a substantially upright storm water drain basin so as to separate the interior space of the drain basin into a first compartment that communicates with an inlet opening of the drain basin and a second compartment that communicates with an outlet opening of the drain basin. A predetermined primary flow path is defined in the baffle number between the first and second compartments near a lower end of the baffle member. Treatment material is positioned, optionally in cartridge form, in a lower end of the second compartment so that storm water in the primary flow path will be directed through the treatment material in order to remediate possible contamination of the storm water. The system is designed in order to facilitate retrofitting of existing storm water drainage infrastructure and for original installation. A method of treating storm water is also disclosed.