We provide a theoretical description of frustrated multiferroic MnI ₂ with a spiral magnetic ordering in magnetic field h. We demonstrate that subtle interplay of exchange coupling, dipolar forces, hexagonal anisotropy, and the Zeeman energy account for the main experimental findings observed recently in this material (Kurumaji et al., Phys. Rev. Lett. 106 (2011) 167206). We describe qualitatively the non-trivial evolution of electric polarization P upon h rotation, changing P direction upon h increasing, and disappearance of ferroelectricity at h>h _c, where h _c is smaller than the saturation field.