The plethysm coefficient $p(ν, μ, λ)$ is the multiplicity of the Schur function $s_λ$ in the plethysm product $s_ν\circ s_μ$. In this paper we use Schur--Weyl duality between wreath products of symmetric groups and the ramified partition algebra to interpret an arbitrary plethysm coefficient as the multiplicity of an appropriate composition factor in the restriction of a module for the ramified partition algebra to the partition algebra. This result implies new stability phenomenon for plethysm coefficients when the first parts of $ν$, $μ$ and $λ$ are all large. In particular, it gives the first positive formula in the case when $ν$ and $λ$ are arbitrary and $μ$ has one part. Corollaries include new explicit positive formulae and combinatorial interpretations for the plethysm coefficients $p((n-b,b), (m), (mn-r,r))$, and $p((n-b,1^b), (m), (mn-r,r))$ when $m$ and $n$ are large.