Window insulating device, kit and system

An interior storm or insulating window, kit for making, and system for utilizing, which window is adjustable in length and width so as to fit different sized window frames, and includes rails around the perimeter with a groove for attaching a retainer strip, the rail further including an engaging cavity to joining to other components, corner brackets for joining rails, a flexible plastic film and a retainer for retaining the flexible plastic film within the groove and for retaining the window within a window frame by use of a flexible spline.