  By utilizing symmetry analysis and electronic structure calculations, we
investigated the low-temperature orthorhombic phase of Ag$_{2}$Se in ${\cal
SG}$~17. In addition to the discovery of a nodal plane at $k_{z}=\pi$ protected
by the joint operation of time-reversal (${\cal T}$) and the 2-fold screw
rotation $S_{2z}$, we found 24 Weyl points mainly residing at the $k_{y}=0$
plane with notable Fermi arc and large quasiparticle interference pattern
(QPI). Due to the absence of mirror symmetry, a pair of Weyl points with
opposite chirality reside at different binding energies, which makes this
system an excellent material candidate for realizing the novel chiral anomaly
related phenomenon, such as the quantized circular photogalvanic and the chiral
magnetic effects. Furthermore, we also reveal the striking spin textures at
$k_{z}=\pi$ plane which demonstrates, in a large region of the surface
Brillouin Zone, a direction-selective spin polarization, which has a strong
implication to spintronic applications.
