  We present an analysis of the first Kepler K2 mission observations of a
rapidly oscillating Ap (roAp) star, HD 24355 ($V=9.65$). The star was
discovered in SuperWASP broadband photometry with a frequency of 224.31
d$^{-1}$, (2596.18 $\mu$Hz; $P = 6.4$ min) and an amplitude of 1.51 mmag, with
later spectroscopic analysis of low-resolution spectra showing HD 24355 to be
an A5 Vp SrEu star. The high precision K2 data allow us to identify 13
rotationally split sidelobes to the main pulsation frequency of HD 24355. This
number of sidelobes combined with an unusual rotational phase variation show
this star to be the most distorted quadrupole roAp pulsator yet observed. In
modelling this star, we are able to reproduce well the amplitude modulation of
the pulsation, and find a close match to the unusual phase variations. We show
this star to have a pulsation frequency higher than the critical cut-off
frequency. This is currently the only roAp star observed with the Kepler
spacecraft in Short Cadence mode that has a photometric amplitude detectable
from the ground, thus allowing comparison between the mmag amplitude
ground-based targets and the $\mu$mag spaced-based discoveries. No further
pulsation modes are identified in the K2 data, showing this star to be a
single-mode pulsator.
