We extend explicit model predictive control (MPC) rigorously to linear distributed parameter systems (DPS). In a theoretical discussion on the foundations of explicit MPC with Hilbert-space optimization, we sort out some of the frequent misconceptions in the present explicit MPC literature. Based on our main result, and a result that it is enough to search among candidate active sets that satisfy the linear independence constraint qualification (LICQ), we give a novel active-set algorithm that implements very fast region-free explicit MPC for highdimensional plants with a relatively long optimization horizon. If the Slater condition is met, then there exists an active set which characterizes optimality, and in this case the algorithm is guaranteed to find it in finite time. We use a Timoshenko beam with input and state constraints to demonstrate the efficacy of the proposed design and the capability of controlling a continuoustime hyperbolic PDE with a discrete-time explicit MPC controller.