A tonneau cover apparatus for securing a flexible cover about a cargo box of a pickup truck. The apparatus comprises the cover and a support frame for attachment to the cargo box. The frame has two opposing side rails, each of which has a lock engaging surface. The support frame further includes a pair of securing plate engagement members, each of which is secured to one of the respective opposing side rails. The apparatus further comprises a flexible cover securing plate attached to the cover. The flexible cover securing plate includes a channel and least one locking member. Each locking member is secured within the channel and each respective locking member is engagable with the lock engaging surface of the respective side rail such that the flexible cover securing plate is prevented from disengaging with respect to the respective side rail when it is in a first position.