You are Eric McClair, dishonorably discharged from FedSun military when you went off to rescue a fallen comrade, thus disobeyed a direct order. The only place that will take you is the Black Knight Legion, a mercenary group headed by Colonel Lawhorn in his name sake mech, the Black Knight. You may have seen combat, but the Black Knight Legion is hardly a normal tour of duty...
Black Knight is an expansion disk for Mechwarrior 4, and requires the original to play. You play an all new single-player campaign where you will visit Kentares IV (the setting of the original Mechwarrior 4). You get to fight and pilot five new mechs (Black Knight, Sunder, Ryoken, Uller, and Wolfhound). You get some new weapons as well, like X-Pulse lasers, large autocannons, and more. There are also new multiplayer modes. The best feature, however, is "black market" in the single-player campaign. You can use the black market to trade in mechs and weapons you don't want for other mechs and weapons that you do want.
You get to kill "yourself" in this game... In MechWarrior 4: Vengeance, you were playing as Ian Dresari. In Black Knight, one of the missions was to kill Ian Dresari, while other missions deal with his lancemates.
As in MW4, FASA Interactive made its way into this game as well. This time, the building is classified as a neutral!