  We present an observational study of the luminous red nova (LRN) AT\,2021biy
in the nearby galaxy NGC\,4631. The field of the object was routinely imaged
during the pre-eruptive stage by synoptic surveys, but the transient was
detected only at a few epochs from $\sim 231$\,days before maximum brightness.
The LRN outburst was monitored with unprecedented cadence both photometrically
and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a
bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by
the longest plateau among LRNe to date, with a duration of 210\,days. A
late-time hump in the light curve was also observed, possibly produced by a
shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of
LRNe. Early-time spectra are characterised by a blue continuum and prominent H
emission lines. Then, the continuum becomes redder, resembling that of a K-type
star with a forest of metal absorption lines during the plateau phase. Finally,
late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K)
with molecular features (e.g., TiO) resembling those of M-type stars.
Spectropolarimetric analysis indicates that AT\,2021biy has local dust
properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of
archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a
$\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) =
5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above
luminosity and colour match those of a luminous yellow supergiant. Most likely,
this source is a close binary, with a 17--24\,\msun\ primary component.
