Doki Doki SOS Brigade
Most mods install on top of a fresh copy of DDLC, free from ddlc.moe. Follow the author's instructions, and keep a clean copy of the game.
About this mod
The basic plot is that Monika will have to deal with another girl that has the power to manipulate reality. (Based on the Anime: The Melancholy of Haruhi Suzumiya)