We present new Multi Unit Spectroscopic Explorer observations of quasar field Q2131-1207 with a log N(H I} = 19.50 ± 0.15 sub-damped Lyman α at z_abs = 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 ± 0.02 and 8.32 ± 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 ± 0.20). For the closest object, a detailed morphokinematic analysis indicates that it is an inclined large rotating disc with V_max = 200 ± 3 km s^-1. We measure the masses to be M_dyn = 7.4 ± 0.4 × 10¹⁰ M_⊙ and M_halo = 2.9 ± 0.2 × 10¹² 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° ± 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.