Detailed observations show that the first ultraluminous X-ray source detected in our neighboring Andromeda galaxy is due to a stellar mass black hole swallowing material at very high rates. The emission of the ultraluminous source probably originates from a system similar to X-ray binaries in our galaxy with matter accreting onto a black hole, which is at least 13 times more massive than our Sun. Unlike X-ray binaries in our own Milky Way, however, this source is much less obscured by interstellar gas and dust, allowing detailed investigations also at low X-ray energies.