/usr/bin/hp-plugin-ubuntu is in hplip 3.12.2-1ubuntu3.5.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | #!/bin/sh
if [ -z $DISPLAY ]; then
    sudo -p \
	"
HP Printer Driver Plug-in Installetion
--------------------------------------
This program will install the proprietary plug-in for HP's printer
driver framework HPLIP into your system. This requires administrator
access. Please enter your password.
User: %u
Password: " \
    hp-plugin -i
else
    if python -c 'import PyQt4.QtGui' 2>/dev/null && which hp-toolbox 2>/dev/null; then
	gksu -D 'HP Printer Driver Plug-in Installation' -- hp-plugin -u >/dev/null 2>&1
	exit 0
    else
	gksu -D 'HP Printer Driver Plug-in Installation' -- xterm -T 'HP Printer Driver Plug-in Installation' -sb -rightbar -hold -e hp-plugin -i >/dev/null 2>&1 &
    fi
fi
 |