  Let $k$ be a commutative ring with identity. A {\it $k$-plethory} is a
commutative $k$-algebra $P$ together with a comonad structure $W_P$, called the
{\it $P$-Witt ring} functor, on the covariant functor that it represents. We
say that a $k$-plethory $P$ is {\it idempotent} if the command $W_P$ is
idempotent, or equivalently if the map from the trivial $k$-plethory $k[e]$ to
$P$ is a $k$-plethory epimorphism. We prove several results on idempotent
plethories. We also study the $k$-plethories contained in $K[e]$, where $K$ is
the total quotient ring of $k$, which are necessarily idempotent and contained
in $\operatorname{Int}(k) = \{f \in K[e]: f(k) \subseteq k\}$. For example, for
any ring $l$ between $k$ and $K$ we find necessary and sufficient
conditions---all of which hold if $k$ is a integral domain of Krull type---so
that the ring $\operatorname{Int}_l(k) = \operatorname{Int}(k) \cap l[e]$ has
the structure, necessarily unique and idempotent, of a $k$-plethory with unit
given by the inclusion $k[e] \longrightarrow \operatorname{Int}_l(k)$. Our
results, when applied to the binomial plethory $\operatorname{Int}({\mathbb
Z})$, specialize to known results on binomial rings.
