Plastic container (110), including a side wall (120) and a flexible portion (132) of a base (130). The flexible portion of the base deflects when the sealed plastic container experiences a differential pressure. The deflection of the flexible portion of the base acts to change the internal volume of the container and thereby reduce the differential pressure. The container is a retortable container. The container may be injection-molded with inner and outer plastic layers (460,462) and a core layer (464) between the inner and outer plastic layers.