The reaction of 2-carboxybenzaldehyde and hydrazine hydrate unexpectedly yielded the title compound, C16H10N2O3, which comprises one phthalide ring, one phthalazine system and a chiral centre. The phthalide unit is almost perpendicular to the phthalazine system, forming a dihedral angle of 87.1 (3)°. The packing is governed by weak C—H...O hydrogen-bonding interactions, forming layers parallel to the ab plane.