Three-dimensional surface-tension-driven Bénard convection at zero Prandtl number is computed in the smallest possible doubly periodic rectangular domain that is compatible with the hexagonal flow structure at the linear stability threshold of the quiescent state. Upon increasing the Marangoni number beyond this threshold, the initially stationary flow becomes quickly time dependent. We investigate the transition to chaos for the case of a free-slip bottom wall by means of an analysis of the kinetic energy time series. We observe a period-doubling scenario for the transition to chaos of the energy attractor, intermittent behavior of a component of the mean velocity field, three characteristic energy levels, and two frequencies that contain a considerable amount of the power spectral density connected with the kinetic energy time series.