  Many of the Kepler close binaries are suggested to constitute hierarchical
triple systems through their eclipse timing variations (ETVs). Eclipses by the
third body in those systems, if observed, provide precise constraints on its
physical and orbital properties, which are otherwise difficult to obtain. In
this Letter, we analyze such a "tertiary event" observed only once in the KIC
6543674 system. The system consists of a short-period ($2.4\,\mathrm{days}$)
inner eclipsing binary and a third body on a wide ($1100\,\mathrm{days}$) and
eccentric ($e\simeq0.6$) orbit. Analysis of three tertiary eclipses around a
single inferior conjunction of the third body yields the mutual inclination
between the inner and outer binary planes to be $3.3^\circ\pm0.6^\circ$,
indicating an extremely flat geometry. Furthermore, combining the timings and
shapes of the tertiary eclipses with the phase curve and ETVs of the inner
binary, we determine the mass and radius ratios of all three bodies in the
system using the Kepler photometry alone. With the primary mass and temperature
from the Kepler Input Catalog, the absolute masses, radii, and effective
temperatures of the three stars are obtained as follows:
$M_\mathrm{A}=1.2\pm0.3\,M_\odot$, $R_\mathrm{A}=1.8\pm0.1\,R_\odot$,
$M_\mathrm{B}=1.1_{-0.2}^{+0.3}\,M_\odot$, $R_\mathrm{B}=1.4\pm0.1\,R_\odot$,
$M_\mathrm{C}=0.50_{-0.08}^{+0.07}\,M_\odot$,
$R_\mathrm{C}=0.50\pm0.04\,R_\odot$, $T_\mathrm{A} \simeq T_\mathrm{B}\simeq
6100\,\mathrm{K}$, and $T_\mathrm{C}<5000\,\mathrm{K}$. Implication for the
formation scenario of close binaries is briefly discussed.
