Every now and then, I see someone wanting to convert a plot file back into a .DWG file. Unfortunately, this is a poor substitute for having the proper backup for your DWG file. Plot files only contain enough information for the plotter to draw the graphics that were plotted.
Having said that, there are programs that will create a DXF or DWG file from your plot file, but keep in mind that depending on the software used for the conversion, the output DXF/DWG will most likely only contain polylines. No arcs, dimensions, circles, text, etc., and no non-graphical information such as linetypes, block definitions, textstyles, layers, tablestyles, dimstyles, etc. either.
First off, PLT is a common file extension for plot files, but it is not a file format. Before you get started, it will help if you know the format of your plot file.
HPGL (Hewlett-Packard Graphics Language) is an older format used with many pen plotters. Most current HP plot drivers (and many other brands) produce HPGL/2 plot files, not HPGL.
Autodesk Express Tools provides a tool to convert HPGL format plot files. It does not work with HPGL/2 files.
HPGL2CAD can produce a DWG or DXF file from an HPGL/2 plot file. You can specify the DWG/DXF version, from AutoCAD r9 to r2000.
Here are some other tools designed for creating DWG/DXF files from plot files.