A merchant crosses your path -
"I've got nothing, I tell ya."
The legendary marine enters a pipe, you can figure out the rest; do you?.
This mod uses a custom Doom2.wad file.
For the best gameplay, use prboom+, you can download it from here: https://sourceforge.net/projects/prboom-plus/
to play, extract the prboom+ then extract both game files, Doom2.wad and SDB.wad on the prboom+ folder and finally drag and drop the SDB.wad file on the prboom-plus.exe executable.