Plethysm is defined in Ian MacDonald's book "Symmetric Functions and Hall Polynomials" (1979, 1995).
Let $f, g$ be polynomials, and write $g$ as a sum of monomials:
$$ g = \sum_\alpha u_\alpha x^\alpha . $$
Now introduce the set of fictitious variables $y_i$ defined by:
$$ \prod (1+ y_i t) = \prod_\alpha (1 + x^\alpha t)^{u_\alpha} $$
and define
$$ f \circ g = f(y_1, y_2, ...) . $$
Another way of calling a plethysm is "composition".
Is this the same as substituting $g$ into $f$?