plain environment, in which a Plain TeX
document may be processed:
\begin{plain}
\input{plain-doc}
\end{plain}
The package is known to fail, for example, with documents that use
AMSTeX; no doubt it would also fail if asked to load Eplain.
All these things can be overcome (although it’s not often easy), but
the environment saves a lot of work on many occasions.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=inclplain