E. Killing lives in a 4D simulation of the 3D world. It's a recording – everything has already happened. Her device, The Horizon, reconstructs her world in time slices, like a slit-scan camera. She needs to find her way out of the simulator.
Horizon is a 4D puzzle game. Every scene in it is a pre-recorded 3D physical reality over a period of time. As Killing moves through the game, she uses the Horizon to select a shift in time to solve her problem – From her perspective it just looks like slit imaging, but she's actually forming a 3D physical space. When she spots a brick falling from an abandoned building, E. Killing stretches that point in time, extending her perspective from the brick's drop until impact. She has just created a staircase she can climb. Construction of Horizon required building a new game engine, called the Unruh Engine, which allows rapid recollection of pre-recorded geometry in 4 dimensions onto a coherent 3D scene with all of the usual game mechanics.