  We report on multiwavelength studies of a blazar NVSS J171822+423948, which
is identified as the low-energy counterpart of 4FGL J1718.5+4237, the unique
$\gamma$-ray source known to be cospatial with the IceCube neutrino event
IC-201221A. After a 12-year long quiescent period undetected by Fermi-LAT,
$\gamma$-ray activities with a tenfold flux increase emerge soon (a few tens of
days) after arrival of the neutrino. Associated optical flares in the ZTF $g$,
$r$, and $i$ bands are observed together with elevated WISE infrared fluxes.
Synchronized variations suggest that both the $\gamma$-ray emission and the
neutrino event are connected to the blazar. Furthermore, the optical spectrum
reveals emission lines at a redshift $z$ = 2.68 $\pm$ 0.01. Thus, it is the
first candidate for a neutrino-emitting blazar at the redshift above 2.
Discussions of theoretical constraints of neutrino production and comparisons
with other candidates are presented.
