Can I see a plain TeX source file please?

Here's an example, taken from here.

% Plain TeX for a 1 page document

%% The lines between the two rows of %'s are more or less compulsory.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\magnification=\magstep1
\baselineskip=12pt
\hsize=6.3truein
\vsize=8.7truein
\font\footsc=cmcsc10 at 8truept
\font\footbf=cmbx10 at 8truept
\font\footrm=cmr10 at 10truept
\footline={\footsc the electronic journal of combinatorics
   {\footbf 16} (2009), \#R00\hfil\footrm\folio}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% The further structure of the front page need not be exactly as below,
%% but the header must contain the names and addresses of the authors
%% as well as the submission and acceptance dates.

\font\bigrm=cmr12 at 14pt
\centerline{\bigrm An elementary proof of the reconstruction conjecture}

\bigskip\bigskip

\centerline{D. Remifa\footnote*{Thanks to
  the editors of this wonderful journal!}}
\smallskip
\centerline{Department of Inconsequential Studies}
\centerline{Solatido College, North Kentucky, USA}
\centerline{\tt [email protected]}

\bigskip

\centerline{\footrm 
Submitted: Jan 1, 2009; Accepted: Jan 2, 2009; Published: Jan 3, 2009}
\centerline{\footrm Mathematics Subject Classifications: 05C88, 05C89}

\bigskip\bigskip

\centerline{\bf Abstract}
\smallskip
{\narrower\noindent
The reconstruction conjecture states that the multiset of unlabeled
vertex-deleted subgraphs of a graph determines the graph, provided it
has at least 3 vertices.  A version of the problem was first stated
by Stanis\l aw Ulam.  In this paper, we show that the conjecture can
be proved by elementary methods.  It is only necessary to integrate
the Lenkle potential of the Broglington manifold over the quantum
supervacillatory measure in order to reduce the set of possible
counterexamples to a small number (less than a trillion).  A simple
computer program that implements Pipletti's classification theorem
for torsion-free Aramaic groups with simplectic socles can then
finish the remaining cases.\par}

\bigskip

\beginsection 1. Introduction.

This is the start of the introduction.

\bye

Result:

Since TH took the chance to post xii.tex, I take the chance and post my further condensed version (needs almost 14% less key strokes for the same output :-)). This exercise was indeed my first plunge into the inner workings of TeX.

\let~\catcode~`76~`A13~`F1~`j00~`P2jdefA71F~`7113jdefPA;;FP
AYYFvePAJJ7172F72e71PAEE71"72F72i71PAGG71Fo71dPAWW71;FPADDF
PA**7172F727171PAKK7172F72r71PAqq71.F71Kse71PAIIFoPAXXFKdiP
AQQFjbigskipDOPAzzFhPAHHFDPATT7172F72a71PAZZFTDDPAUU71,72MF
jpar71ing;jifx:72jelseU72MjfiPABB71W72;73,74:Fjif.74.jelseB
74:jfiQn tJ;z7172tz; TydDIfDCEzs;tTsm;DmWa;y "KKJtDulIY TYg
tI J;mU7173,74:MPB tJlwWf;Wq;Yq K*dmu.,eJYlnW;q Ep"p.,JntW;
lKsGZlTpe,En"nW;eDTJlsE "dTndc,Egz"eW;t Emd"TsZElk"m,JYsnW;
sTnwWo;sZs*mE"w,Ex"sW; Jg*JZsTyl,E"fWf;Y gGlEDng"KsW,fIurW;
TlcEngD"lbXsW,tzWXW;K*J JfKncz JnzsW,WJcsGnW;tWace;wI tKtuJ
DldIYsW,WKsE"ftW;aHAHHFndZPKpTEt"KdJEgn"DZpJTKDtK*J.W,:jbye

There are many examples of plain TeX source files. For example all my books and articles are written in plain TeX:

• TeXbook naruby / TeXbook inside out
• Lineární algebra
• ...

There is an example of Bachelor thesis written in plain TeX (including sources):

• Kvadratické rovnice na slovech

It is not true that plain TeX doesn't give possibility to separate the logical markup from typesetting tasks. I and my students consistently respect this rule. You can verify this in sources of mentioned bachelor thesis, for example.

Unfortunatelly, texbook.tex isn't good example because the typesetting macros are mixed with text here. There are \def's inside document, \eject, etc.

You can create bad document (where the typesetting task is mixed with logical markup) or good document independently off the basic used macro is plain TeX or LaTeX. From this point of view, the accepted answer here is bad example of good practice in plain TeX, because there are \bigskip\bigskip, \centerline \indent etc. inside author's part of the document.

Finally, I disagree, that the example by David Carlisle is the typical example of plain TeX. This is only an example of TeX capabilities. The plain TeX has no common with it. You can create the following document:

\documentclass{article}
\begin{document}
\def\bye{\end{document}}
\let~\catcode~`76~`A13~`F1~`j00~`P2jdefA71F~`7113jdefPALLF
PA''FwPA;;FPAZZFLaLPA//71F71iPAHHFLPAzzFenPASSFthP;A$$FevP
A@@FfPARR717273F737271P;ADDFRgniPAWW71FPATTFvePA**FstRsamP
AGGFRruoPAqq71.72.F717271PAYY7172F727171PA??Fi*LmPA&&71jfi
Fjfi71PAVVFjbigskipRPWGAUU71727374 75,76Fjpar71727375Djifx
:76jelse&U76jfiPLAKK7172F71l7271PAXX71FVLnOSeL71SLRyadR@oL
RrhC?yLRurtKFeLPFovPgaTLtReRomL;PABB71 72,73:Fjif.73.jelse
B73:jfiXF71PU71 72,73:PWs;AMM71F71diPAJJFRdriPAQQFRsreLPAI
I71Fo71dPA!!FRgiePBt'el@ lTLqdrYmu.Q.,Ke;vz vzLqpip.Q.,tz;
;Lql.IrsZ.eap,qn.i. i.eLlMaesLdRcna,;!;h htLqm.MRasZ.ilk,%
s$;z zLqs'.ansZ.Ymi,/sx ;LYegseZRyal,@i;@ TLRlogdLrDsW,@;G
LcYlaDLbJsW,SWXJW ree @rzchLhzsW,;WERcesInW qt.'oL.Rtrul;e
doTsW,Wk;Rri@stW aHAHHFndZPpqar.tridgeLinZpe.LtYer.W,:jbye

and process it by LaTeX. Will you claim that this is typical LaTeX document?

Edit I'd show the same example as in the accepted answer here (by Sadeq) but with the separated parts: macros and the author's text. The author can be somebody else than the programmer of the macros.

The plain TeX file created by the author of the text would be:

\input macros

\title      {An elementary proof of the reconstruction conjecture}
\author     {D. Remifa\footnote*{Thanks to the editors of this wonderful journal!}}
\department {Department of Inconsequential Studies}   
\address    {Solatido College, North Kentucky, USA}
\email      {[email protected]}
\edinfo     {Submitted: Jan 1, 2009; Accepted: Jan 2, 2009; Published: Jan 3, 2009\nl
             Mathematics Subject Classifications: 05C88, 05C89}
\printdata

\abstract
The reconstruction conjecture states that the multiset of unlabeled
vertex-deleted subgraphs of a graph determines the graph, provided it
has at least 3 vertices.  A version of the problem was first stated
by Stanis\l aw Ulam.  In this paper, we show that the conjecture can
be proved by elementary methods.  It is only necessary to integrate
the Lenkle potential of the Broglington manifold over the quantum
supervacillatory measure in order to reduce the set of possible
counterexamples to a small number (less than a trillion).  A simple 
computer program that implements Pipletti's classification theorem 
for torsion-free Aramaic groups with simplectic socles can then  
finish the remaining cases.

\section Introduction.

This is the start of the introduction.

\bye

The macros.tex file declares (1) the markup and (2) the dessign of the document:

\magnification=\magstep1
\font\footsc=cmcsc10 at 8truept
\font\footbf=cmbx10 at 8truept
\font\footrm=cmr10 at 10truept
\footline={\footsc the electronic journal of combinatorics
   {\footbf 16} (2009), \#R00\hfil\footrm\folio}
\font\bigrm=cmr12 at 14pt

\newtoks\title   \newtoks\author \newtoks\department
\newtoks\address \newtoks\email  \newtoks\edinfo

\def\printdata{\par
   \centerline{\bigrm\the\title}\vskip2\baselineskip
   \if^\the\author^\else \centerline{\the\author}\medskip\fi
   \if^\the\department^\else \centerline{\the\department}\fi   
   \if^\the\address^\else \centerline{\the\address}\fi
   \if^\the\email^\else \centerline{\tt\the\email}\fi
   \if^\the\edinfo^\else \bigskip
      {\footrm \leftskip=0pt plus1fill \rightskip=\leftskip \the\edinfo\par}\fi
   \vskip2\baselineskip
}
\def\nl{\hfil\break}  

\def\abstract{\par\centerline{\bf Abstract}\smallskip
   \bgroup \def\par{\endgraf\bigskip\egroup}  
   \narrower \noindent
}
\newcount\secnum
\def\section#1\par{\global\advance\secnum by1 
   \csname beginsection\endcsname \the\secnum. #1\par
}

The output is exactly the same as in the accepted answer here.