The package plain.sty defines a LaTeX environment which can process a document written to use Knuth's basic Plain TeX macros.

Inside the environment, not everything is as Knuth intended (some Plain commands are suppressed, some are faked, and some basic mechanisms—such as font selection and the output routine—remain as in LaTeX). Thus “simple” documents will run, but elaborate macros defined in the document have a chance of failing.

The author is David Carlisle.