Abstract: An enclosure assembly enabling the selective covering of an area such as the bed of a pickup truck of the like. The assembly includes a plurality of extendable and collapsible cover sections interconnected by an actuating system. The actuating system enables the rear cover section of the assembly to be pivoted to a collapsed or extended position by a pivoting structure having unique cam and leverage characteristics. The entire assembly can be selectively power operated or locked into position using a clutch mechanism associated with the actuation system. The enclosure assembly affords simple and effective operation to provide for the convenient use of the truck bed or similar area.