Understanding the dynamics of correlated many-body quantum systems is a challenge for modern physics. Owing to the simplicity of theirHamiltonians, He-4 (bosons) and He-3 (fermions) have served as model systems for strongly interacting quantum fluids, with substantial efforts devoted to their understanding. An important milestone was the direct observation of the collective phonon-roton mode in liquid He-4 by neutron scattering, verifying Landau's prediction(1) and his fruitful concept of elementary excitations. In a Fermi system, collective density fluctuations (known as 'zero-sound' in He-3, and 'plasmons' in charged systems) and incoherent particle-hole excitations are observed. At small wavevectors and energies, both types of excitation are described by Landau's theory of Fermi liquids(2,3). At higher wavevectors, the collectivemode enters the particle-hole band, where it is strongly damped. The dynamics of Fermi liquids at high wavevectors was thus believed to be essentially incoherent. Here we report inelastic neutron scattering measurements of a monolayer of liquid He-3, observing a roton-like excitation. We find that the collective density mode reappears as a well defined excitation atmomentum transfers larger than twice the Fermi momentum. We thus observe unexpected collective behaviour of a Fermi many-body system in the regime beyond the scope of Landau's theory. A satisfactory interpretation of the measured spectra is obtained using a dynamic many-body theory(4).