The radio source 1642 + 690 (4C 69.21) has previously been found to contain a dominant, compact core with a jet extending 5 arcsec to the south. This paper presents two 5 GHz VLBI maps of the core, made in 1980 and 1983, which show that the milliarcsecond structure of the source is similar to that of the well-known superluminal sources 3C 273 and 3C 345. A milliarcsecond jet is closely aligned with the arcsecond jet, and a 'knot' in the jet is moving outwards along the jet at 0.34 mas/yr. It is found that the associated optical object is a quasar with redshift z = 0.75. At this redshift, the displacement of the knot implies an apparent transverse velocity of 9.3/h c (H0 = 100h km/s Mpc, q0 = 0.05).