  M31 has experienced a recent tumultuous merger history as evidenced from the
many substructures that are still present in its inner halo, particularly the
G1-Clump, NE- and W- shelves, and the Giant Stream (GS). We present planetary
nebulae (PNe) line-of-sight velocity (LOSV) measurements covering the entire
spatial extent of these four substructures. We further use predictions for the
satellite and host stellar particle phase space distributions for a major
merger (mass ratio = 1:4) simulation to help interpret the data. The measured
PN LOSVs for the two shelves and GS are consistent with those from red giant
branch stars. Their projected radius vs. LOSV phase space, links the formation
of these substructures in a single unique event, consistent with a major
merger. We find the G1-clump to be dynamically cold compared to the M31 disc
($\rm\sigma_{LOS, PN}=27$ km s$^{-1}$), consistent with pre-merger disc
material. Such a structure can not form in a minor merger (mass ratio
$\sim$1:20), and is therefore a smoking gun for the recent major merger event
in M31. The simulation also predicts the formation of a predominantly in-situ
halo from splashed-out pre-merger disc material, in qualitative agreement with
observations of a metal-rich inner halo in M31. Juxtaposed with previous
results for its discs, we conclude that M31 has had a recent (2.5 - 4 Gyr ago)
`wet' major merger with the satellite falling along the GS, heating the
pre-merger disc to form the M31 thicker disc, rebuilding the M31 thin disc, and
creating the aforementioned inner-halo substructures.
