The motion of particles displaced by a Gravitational Wave could provide a way to detect the latter. Following Souriau (1973), the geodesic equations can be integrated using the 5-parameter isometries of plane gravitational waves, identified as Lévy-Leblond's (1965) ``Carroll'' group in 2+1 dimensions with no rotations. The group acts as a symmetry for the particles. The associated conserved quantities determine the trajectories, which are consistent with the velocity effect. (Based on joint work with C. Duval, G. W. Gibbons, M. Elbistan and P. M. Zhang)