The fangame starts off with a scene including Sans, Papyrus and Undyne. (Still UNDERTALE) You have to talk to them to go on. Then you advance into the ruins and talk to Toriel. She also talks to you. Then you go on through the ruins until you reach flowey. You talk to him, then something weird happens...
The fangame truly starts with the dialogue, "You seem to have fallen down... Again?" Then you ask flowey what happened. Flowey says that he didn't do it, then he mentions the idea that something or someone is overridding Frisk's determination. You advance through the RUINS as usual, but then you encounter a Loox. After 2 hits, in red text, the box says, "Ha Ha!", then you kill the Loox. You go on until you reach a certain point in the ruins and you take control of Sans. Sans is patrolling until he reaches Papyrus's post. After that, you are back in control. You reach sans, and then he tells you to hide behind him. Then Papyrus appears and asks Sans about his patrol. He tells papyrus his eyes were playing tricks on him and then he apologises to papyrus about it. Then you go on and Sans follows you until he mentions the demo's over.
Here is the Fangame:https://gamejolt.com/games/UNDERFELL/196935