  We present a detailed analysis of a superflare on the active M dwarf star AD
Leonis. The event presents a rare case of a stellar flare observed
simultaneously in X-rays (with XMM-Newton) and in optical (with the Transiting
Exoplanet Survey Satellite, TESS). The radiated energy both in the 0.2-12 keV
X-ray band ($1.26 \pm 0.01 \cdot 10^{33}$ erg) and the bolometric value
($E_{F,bol} = 5.57 \pm 0.03 \cdot 10^{33}$ erg) put this event at the lower end
of the superflare class. The exceptional photon statistics deriving from the
proximity of AD Leo has enabled measurements in the 1-8 AA GOES band for the
peak flux (X1445 class) and integrated energy ($E_{F,GOES} = 4.30 \pm 0.05
\cdot 10^{32}$ erg), making possible a direct comparison with data on flares
from our Sun. From extrapolations of empirical relations for solar flares we
estimate that a proton flux of at least $10^5\,{cm^{-2} s^{-1} sr^{-1}}$
accompanied the radiative output. With a time lag of 300s between the peak of
the TESS white-light flare and the GOES band flare peak as well as a clear
Neupert effect this event follows very closely the standard (solar) flare
scenario. Time-resolved spectroscopy during the X-ray flare reveals, in
addition to the time evolution of plasma temperature and emission measure, a
temporary increase of electron density and elemental abundances, and a loop
that extends in the corona by 13% of the stellar radius ($4 \cdot 10^9$ cm).
Independent estimates of the footprint area of the flare from TESS and
XMM-Newton data suggest a high temperature of the optical flare (25000 K), but
we consider more likely that the optical and X-ray flare areas represent
physically distinct regions in the atmosphere of AD Leo.
