  The tidal disruption event (TDE) AT2018fyk has unusual X-ray, UV, and optical
light curves that decay over the first $\sim$600d, rebrighten, and decay again
around 1200d. We explain this behavior as a one-off TDE associated with a
massive black hole (BH) \emph{binary}. The sharp drop-offs from $t^{-5/3}$
power laws at around 600d naturally arise when one BH interrupts the debris
fallback onto the other BH. The BH mass $M_\bullet$ derived from fitting X-ray
spectra with a slim disk accretion model and, independently, from fitting the
early UV/optical light curves, is smaller by two orders of magnitude than
predicted from the $M_\bullet$--$\sigma_*$ host galaxy relation, suggesting
that the debris is accreted onto the secondary, with fallback cut off by the
primary. Furthermore, if the rebrightening were associated with the primary, it
should occur around 5000d, not the observed 1200d. The secondary's mass and
dimensionless spin is $M_{\bullet,{\rm s}}=2.7^{+0.5}_{-1.5} \times 10^5
M_\odot$ and $a_{\bullet,{\rm s}}>0.3$ (X-ray spectral fitting), while the
primary's mass is $M_{\bullet,{\rm p}}=10^{7.7\pm0.4}M_\odot$
($M_\bullet$-$\sigma_*$ relation). An intermediate mass BH secondary is
consistent with the observed UV/optical light curve decay, i.e., the
secondary's outer accretion disk is too faint to produce a detectable emission
floor. The time of the first accretion cutoff constrains the binary separation
to be $(6.7\pm 1.2) \times 10^{-3}~{\rm pc}$. X-ray spectral fitting and timing
analysis indicate that the hard X-rays arise from a corona above the
secondary's disk. The early UV/optical emission, suggesting a super-Eddington
phase for the secondary, possibly originates from shocks arising from debris
circularization.
