Chemical Plant might aswell be considered two levels of the original game that you never played. It strongly sticks to the base theme and is just as good as ID's work.
Your mission is the typical one - enter the Strogg base (this time a Chemical Plant) and destroy the reaction vessel that is deep underground. After a strictly-base first level with crates everywhere, you are taken down to the caves and lava that creeps into the underground base. Level design was very good with some nicely lit areas. Layout was also good, usually the loop method is used, where you eventually end up back at a point you passed earlier but can now open the door and proceed on.
Gameplay was well done, although on normal it is doubtful that you'll ever get below 100% health! If you like the 'run in and blast everything away' style, then these two levels are for you. There are mainly Soldiers and Infantry to fight, and you'll only need to use the Dbl-Shotgun and Machinegun, with a few grenades to take down the Commanders. The only let-down of these two levels was the end battle. The last thing you end up killing is a pathetic fish (if your nice and don't kill your friend kenny).
Otherwise an extremely fun level set that will keep you happy for about half an hour... and then you'll probably play it again!