  It is widely assumed that a star and its protoplanetary disk are initially
aligned, with the stellar equator parallel to the disk plane. When observations
reveal a misalignment between stellar rotation and the orbital motion of a
planet, the usual interpretation is that the initial alignment was upset by
gravitational perturbations that took place after planet formation. Most of the
previously known misalignments involve isolated hot Jupiters, for which
planet-planet scattering or secular effects from a wider-orbiting planet are
the leading explanations. In theory, star/disk misalignments can result from
turbulence during star formation or the gravitational torque of a wide-orbiting
companion star, but no definite examples of this scenario are known. An ideal
example would combine a coplanar system of multiple planets -- ruling out
planet-planet scattering or other disruptive post-formation events -- with a
backward-rotating star, a condition that is easier to obtain from a primordial
misalignment than from post-formation perturbations. There are two previously
known examples of a misaligned star in a coplanar multi-planet system, but in
neither case has a suitable companion star been identified, nor is the stellar
rotation known to be retrograde. Here, we show that the star K2-290 A is tilted
by $124\pm 6$ degrees compared to the orbits of both of its known planets, and
has a wide-orbiting stellar companion that is capable of having tilted the
protoplanetary disk. The system provides the clearest demonstration that stars
and protoplanetary disks can become grossly misaligned due to the gravitational
torque from a neighbouring star.
