We use two input beams (control and probe) in the same plane of incidence to study second-harmonic generation from achiral thin films with in-plane isotropy. Appropriate linear control polarizations break the reflection symmetry of the setup and lead to different second-harmonic efficiency for left-and right-hand circular probe polarizations. Circular control polarizations are described by the spin vector, which is also in the plane of incidence and apparently does not break the symmetry. Because of spin being an axial vector, the symmetry is nevertheless broken and optical activity observed. [S0031-9007(99)09088-2].