We present and disucss observations of a flare event on the dM4.5e star AT Mic (Gl 799AB). These data show evidence of an energetic flare with a radiated energy of 3 10$^{31}$ erg in the range 3600-4200$\AA$. The Balmer emission lines display a prominent blue asymmetry which has been used to constrain the velocity of the associated bulk plasma motion. Simple models were applied to derive order of magnitude estimates for the mass and kinetic energy budget of the flare for assumed electron density, ionization fraction and temperature values. It is found that the kinetic energy of the flare plasma is two orders of magnitude less than the radiative energy release. The chromospheric Ca II H \& K lines are also observed to have a blue-shifted component which has not been observed prior to these results. We suggest that this is evidence of a very energetic particle beam penetrating to the cooler atmospheric layers where Ca II H \& K is formed. Key words: stars: flares - stars: UV ceti-type - stars: magetic activity