  The BL Lac PKS 1413+135 was observed by the Large Survey Project "MeerKAT
Absorption Line Survey" (MALS) in the L-band, at 1139 MHz and 1293-1379 MHz,
targeting the HI and OH lines in absorption at z = 0.24671. The radio continuum
is thought to come from a background object at redshift lower than 0.5, as
suggested by the absence of gravitational images. The HI absorption line is
detected at high signal-to-noise, with a narrow central component, and a red
wing, confirming previous results. The OH 1720 MHz line is clearly detected in
(maser) emission, peaking at a velocity shifted by -10 to -15 km/s with respect
to the HI peak. The 1612 MHz line is lost due to radio interferences. The OH
1667 MHz main line is tentatively detected in absorption, but not the 1665 MHz
one. Over 30 years, a high variability is observed in optical depths, due to
the rapid changes of the line of sight, caused by the superluminal motions of
the radio knots. The HI line has varied by 20 per cent in depth, while the
OH-1720 MHz depth has varied by a factor 4. The position of the central
velocity and the widths also varied. The absorbing galaxy is an early-type
spiral (maybe S0) seen edge-on, with a prominent dust lane, covering the whole
disk. Given the measured mass concentration, and the radio continuum size at
centimeter wavelengths (100 mas corresponding to 400 pc at z = 0.25), the width
of absorption lines from the nuclear regions are expected up to 250 km/S. The
narrowness of the observed lines (< 15 km/s) suggest that the absorption comes
from an outer gas ring, as frequently observed in S0 galaxies. The millimetric
lines are even narrower (< 1 km/s), which corresponds to the continuum size
restricted to the core. The core source is covered by individual 1 pc molecular
clouds, of column density a few 10^22 cm-2, which is compatible with the gas
screen detected in X-rays.
