The BL Lac PKS 1413+135 was observed by the Large Survey Project "MeerKAT Absorption Line Survey" (MALS) in the L-band, at 1139MHz and 1293-1379MHz, 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 1720MHz line is clearly detected in (maser) emission, peaking at a velocity shifted by -10 to -15km/s with respect to the HI peak. The 1612MHz line is lost due to radio interferences. The OH 1667MHz main line is tentatively detected in absorption, but not the 1665MHz 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% in depth, while the OH-1720MHz 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 400pc at z=0.25), the width of absorption lines from the nuclear regions are expected up to 250km/s. The narrowness of the observed lines (<15km/s) suggest that the absorption comes from an outer gas ring, as frequently observed in S0 galaxies. The millimetric lines are even narrower (<1km/s), which corresponds to the continuum size restricted to the core. The core source is covered by individual 1pc molecular clouds, of column density a few 10^22^cm^-2^, which is compatible with the gas screen detected in X-rays.