We describe explicitly all quaternionic contact hypersurfaces (qc-hypersurfaces) in the flat quaternion space Hn+1 and the quaternion projective space. We show that up to a quaternionic affine transformation a qc-hypersurface in Hn+1 is contained in one of the three qc-hyperquadrics in Hn+1. Moreover, we show that an embedded qc-hypersurface in a hyper-Kähler manifold is qc-conformal to a qc-Einstein space and the Riemannian curvature tensor of the ambient hyper-Kähler metric is degenerate along the hypersurface.