  We propose a relativistic gravitational theory leading to modified Newtonian
dynamics, a paradigm that explains the observed universal galactic acceleration
scale and related phenomenology. We discuss phenomenological requirements
leading to its construction and demonstrate its agreement with the observed
cosmic microwave background and matter power spectra on linear cosmological
scales. We show that its action expanded to second order is free of ghost
instabilities and discuss its possible embedding in a more fundamental theory.
