1)since the Linux Mint Debian 1- we are using is still not "stable", we do get to see a few glitches like the one you have reg. plymouth.
2) Open synaptic Manager using your root password. Click on "Search"and you will see A) Search-->type plymouth and B) Under"Look in--> type "Name and Description". A drop down sort of menu with items specific to "plymouth" will be displayed.C) Highlight plymouth,right-click,open properties and open "dependencies".Here, you will find what are the pre-depends,suggests and conflicts!D) Now,open the "Installed files"which is located next to the "Dependencies". Check(if you want to be doubly sure) to see if the predepends are installed,if not install. E) "Suggests" indicate that these dependencies are yet to be installed,so install them!
As a by-note, in properties of plymouth, select "version",you will find "(testing)"
! You should do fine once this job is done!Happy hunting!