Significance The Sun’s equator lines up with the orbits of the planets. This fact supports the theory that stars and their planets inherit their angular momentum from the same source: the gravitational collapse of a molecular cloud. Most astronomers expected spin-orbit alignment to be a universal feature of planetary systems. This proved false: many drastic misalignments are known, and many possible reasons have been offered. In one theory, a distant companion star upsets the alignment at an early stage, while the star is still surrounded by a protoplanetary disk. Here, the K2-290 system is shown to be the best-known candidate for such a primordial misalignment. The star rotates backward, and a companion star with suitable properties has been identified.