  We describe a "super-Earth"-size ($2.30\pm0.15R_{\oplus}$) planet transiting
an early K-type dwarf star in the Campaign 4 field observed by the K2 mission.
The host star, EPIC 210363145, was identified as a member of the approximately
120-Myr-old Pleiades cluster based on its kinematics and photometric distance.
It is rotationally variable and exhibits near-ultraviolet emission consistent
with a Pleiades age, but its rotational period is ~20 d and its spectrum
contains no H$\alpha$ emission nor the Li I absorption expected of Pleiades K
dwarfs. Instead, the star is probably an interloper that is unaffiliated with
the cluster, but younger (< 1 Gyr) than the typical field dwarf. We ruled out a
false positive transit signal produced by confusion with a background eclipsing
binary by adaptive optics imaging and a statistical calculation. Doppler radial
velocity measurements limit the companion mass to <2 times that of Jupiter.
Screening of the lightcurves of 1014 potential Pleiades candidate stars
uncovered no additional planets. An injection-and-recovery experiment using the
K2 Pleiades lightcurves with simulated planets, assuming a planet population
like that in the Kepler prime field, predicts only 0.8-1.8 detections (vs. ~20
in an equivalent Kepler sample). The absence of Pleiades planet detections can
be attributed to the much shorter monitoring time of K2 (80 days vs. 4 years),
increased measurement noise due to spacecraft motion, and the intrinsic
noisiness of the stars.
