  We report the discovery of EPIC 246851721 b, a "tropical" Jupiter in a
6.18-day orbit around the bright ($V=11.439$) star EPIC 246851721 (TYC
1283-739-1). We present a detailed analysis of the system using $K2$ and
ground-based photometry, radial velocities, Doppler tomography and adaptive
optics imaging. From our global models, we infer that the host star is a
rapidly rotating ($v \sin i = 74.92 $ km s$^{-1}$) F dwarf with
$T_\mathrm{eff}$ = 6202 K, $R_\star = 1.586 \ R_\odot$ and $M_\star= 1.317 \
M_\odot$. EPIC 246851721 b has a radius of $1.051 \pm 0.044 R_J$, and a mass of
3.0$^{+1.1}_{-1.2} M_J$ . Doppler tomography reveals an aligned spin-orbit
geometry, with a projected obliquity of $-1.47^{\circ\ +0.87}_{\ -0.86}$,
making EPIC 246851721 the fourth hottest star to host a Jovian planet with $P >
5$ days and a known obliquity. Using quasi-periodic signatures in its light
curve that appear to be spot modulations, we estimate the star's rotation
period, and thereby infer the true obliquity of the system to be $3.7^{\circ\
+3.7}_{\ -1.8}$. We argue that this near-zero obliquity is likely to be
primordial rather than a result of tidal damping. The host star also has a
bound stellar companion, a $0.4 \ M_\odot$ M dwarf at a projected separation of
2100 AU, but the companion is likely incapable of emplacing EPIC 246851721 b in
its current orbit via high eccentricity Kozai-Lidov migration.
