Mortalis is a traditional Yume Nikki fangame. Shimotsuki refuses to leave her room, instead exploring a dream world seeking to collect effects. Additionally, there is a game console resembling a GameCube in the room that can be used to play a minigame called Tomato.

Uniquely, music tracks are not given by merely entering areas. Instead, tracks are collected through finding floating orange music notes in dream world locations.