  The interaction between an accreting satellite and the Andromeda galaxy (M31)
has been studied analytically and numerically, using a high-resolution N-body
simulation with $4\times10^7$ particles. For the first time, we show the
self-gravitating response of the disk, the bulge, and the dark matter halo of
M31 to an accreting satellite. We reproduce the Stream and the shells at the
East and West side of M31, by following the evolution of the collision 4 Gyr
into the future, and show that recently discovered diffuse arcs on the south
side of the minor axis are the remnants of a similar collision that occurred
3-4 Gyr earlier than the Stream event. The present day integrity of the M31
disk constrains the satellite mass to be $M \leq 5\times10^9 M_\odot$. The
stars that were originally in the center of the satellite are now in the east
shell. Therefore, observations in this region might reveal additional clues
about the nature of satellite, such as the central core and any metallicity
gradient.
