We study the focusing 3d cubic NLS equation with $H^1$ data at the
mass-energy threshold, namely, when $M[u_0]E[u_0]{=}M[Q]E[Q]$. In earlier
works of Holmer-Roudenko and Duyckaerts-Holmer-Roudenko,
the behavior of solutions (i.e., scattering and blow up in finite
time) was classified when $M[u_0]E[u_0] < M[Q]E[Q]$. In this paper,
we first exhibit 3 special solutions: $e^{it} Q$ and $Q^\pm$,
where $Q$ is the ground state, $Q^\pm$ exponentially approach the
ground state solution in the positive time direction, $Q^+$ has
finite time blow up and $Q^-$ scatters in the negative time
direction. Secondly, we classify solutions at this threshold and
obtain that up to $\dot{H}^{1/2}$ symmetries, they behave exactly
as the above three special solutions, or scatter and blow up in
both time directions as the solutions below the mass-energy
threshold. These results are obtained by studying the spectral
properties of the linearized Schrödinger operator in this
mass-supercritical case, establishing relevant modulational
stability and careful analysis of the exponentially decaying
solutions to the linearized equation.