We consider a gas of non-interacting fermions that is released from a box into the vacuum and look at the entanglement between the escaped particles with those in the box. This provides a simple analytically tractable model that reproduces many features of the Page curve characterizing the evolution of entanglement entropy during the evaporation of a black hole. Apart from the entropy we consider several other physical observables and show that the framework of generalized hydrodynamics provides a rather surprisingly accurate description of the quantum dynamics.