The interaction of an intense laser field with a thin foil is studied within the framework of a 1D capacitor model with mobile negative point-charges. The model describes the excitation of plane electrostatic oscillations, taking into account the effects of particle–particle crossings and the presence of vacuum boundaries. It is demonstrated how vacuum excursions of particles cause wavebreaking within the foil. From wavebreaking points, charge density discontinuities are shown to propagate further along particle caustics. Absorption is calculated as a function of laser intensity and frequency. It is found that a thin overdense foil can absorb about 5 times more energy than a corresponding surface of an extended solid. This is explained by the doubling of surface charges and by electron heating in the potential well of the foil. A further absorption maximum is obtained below the plasma frequency due to resonance absorption.