Wolfenstein 3D: First Encounter is a shareware mini-campaign episode version of Wolfenstein 3D for the Macintosh, and comprises three levels from the first main game for that platform, Wolfenstein 3D: Second Encounter. It is part of the Mac Family games.
B.J. leaves a path of destruction in his wake as he attempts to escape Castle Wolfenstein. He successfully wastes the boss Hans Grosse during his first encounter with him, proving the Nazi rabble is no match for him and his chaingun.
B.J. had made his way through three levels of Castle Wolfenstein but had not yet escaped. He still needed to get out of the castle and get back home.
B.J. is apparently recaptured, and moved to more secure confines in Castle Dresden which is also under the command of the healed Hans Grosse (for the beginning of the Second Encounter/Original Encounter).
Behind the scenes
Buying the full game added the Second Encounter campaign which comprised a total of 30 levels (including the first 3 from First Encounter). Later, in November 1994, the Third Encounter was released as an additional campaign that could be bought after obtaining Second Encounter. The three levels from First Encounter are also included (without being called "First Encounter") in ports of Second Encounter to other platforms of the Mac Family games.