  We formulate and study an effective Hamiltonian for low-energy Kramers
doublets of $d^1$-ions on a square lattice. We find that the system exhibits a
magnetically hidden order in which the expectation values of the local spin and
orbital moments both vanish. The order parameter responsible for a
time-reversal symmetry breaking has a composite nature and is a spin-orbital
analog of a magnetic octupole. We argue that such a hidden order is realized in
the layered perovskite Sr$_2$VO$_4$.
