  Neutron scattering experiments have been performed on the ternary rare-earth
diborocarbide Ce$^{11}$B$_2$C$_2$. The powder diffraction experiment confirms
formation of a long-range magnetic order at $T_{\rm N} = 7.3$ K, where a
sinusoidally modulated structure is realized with the modulation vector ${\bm
q} = [0.167(3), 0.167(3), 0.114(3)]$. Inelastic excitation spectra in the
paramagnetic phase comprise significantly broad quasielastic and inelastic
peaks centered at $\hbar \omega \approx 0, 8$ and 65 meV.
Crystalline-electric-field (CEF) analysis satisfactorily reproduces the
observed spectra, confirming their CEF origin. The broadness of the
quasielastic peak indicates strong spin fluctuations due to coupling between
localized $4f$ spins and conduction electrons in the paramagnetic phase. A
prominent feature is suppression of the quasielastic fluctuations, and
concomitant growth of a sharp inelastic peak in a low energy region below
$T_{\rm N}$. This suggests dissociation of the conduction and localized $4f$
electrons on ordering, and contrasts the presently observed incommensurate
phase with spin-density-wave order frequently seen in heavy fermion compounds,
such as Ce(Ru$_{1-x}$La$_x$)$_2$Si$_2$.
