Nonlinear evolution of the radiative-condensation instability is investigated in the intermediate-wavelength limit, when the growth rate is maximal. The dynamics of a confined plasma is considered. This paper treats the planar geometry. Nonlinear reduced equations are derived for the instability, which account for the nonlocal feedback resulting from mass conservation and the finite size of the system. For a bistable heating-cooling function, it is shown that, in contrast to a number of previous studies where the isobaricity condition was employed, one-dimensional coherent patterns (spatial coexistence of two locally stable thermal equilibria, cool and hot) persist for a very long time.