A lot of HL2 mods that I've played all require some firearm to get past any enemies I come by. What Research and Development does is strip you of your firearms and equip you with a gravity gun, bug pheromones and more. The opening scene's leaves you questioning where you actually are and how in the heck you got there in the first place. You start off witnessing headcrabs being executed and then returning the favor to some combine.
From the beginning the puzzles are a bit difficult and mix between medium and hard. The visuals have great detail and the high structures give a great depth of field to the surroundings. The puzzles are a delight and the combat even more.