We present new Multi Unit Spectroscopic Explorer observations of quasar field Q2131-1207 with a log N(HI)N(HI) = 19.50 plus or minus 0.15 sub-damped Lyman a at zabs = 0.42980. We detect four galaxies at a redshift consistent with that of the absorber where only one was known before this study. Two of these are star-forming galaxies, while the ones further away from the quasar (>140 kpc) are passive galaxies. We report the metallicities of the H II regions of the closest objects (12 + log(O/H) = 8.98 plus or minus 0.02 and 8.32 plus or minus 0.16) to be higher or equivalent within the errors to the metallicity measured in absorption in the neutral phase of the gas (8.15 plus or minus 0.20). For the closest object, a detailed morphokinematic analysis indicates that it is an inclined large rotating disc with Vmax = 200 plus or minus 3 km s... We measure the masses to be Mdyn = 7.4 plus or minus 0.4 x 10... M... and M sub( halo) = 2.9 plus or minus 0.2 x 10... M... Some of the gas seen in absorption is likely to be corotating with the halo of that object, possibly due to a warped disc. The azimuthal angle between the quasar line-of-sight and the projected major axis of the galaxy on the sky is 12... plus or minus 1... which indicates that some other fraction of the absorbing gas might be associated with accreting gas. This is further supported by the galaxy to gas metallicity difference. Based on the same arguments, we exclude outflows as a possibility to explain the gas in absorption. The four galaxies form a large structure (at least 200 kpc wide) consistent with a filament or a galaxy group so that a fraction of the absorption could be related to intragroup gas. (ProQuest: ... denotes formulae/symbols omitted.)