Plutonium PAK/Atomic Edition: The Atomic Edition of Duke Nukem 3D was released in November 1996, and contained the original 3 episode game as well as a new eleven-level fourth episode. The Plutonium PAK was also released as an upgrade package to convert the original release of Duke Nukem 3D (v1.3d) to the new Atomic Edition (v1.4, later patched to v1.5). It introduces new enemies, a new final boss (The Queen), weapons, and changes to the script to make it easier to mod. Finally, the player can set up a multiplayer session against CPU bots.
Source: Wikipedia, "Duke Nukem 3D", available under the CC-BY-SA License.