It starts with the game's protagonist, Gamma-Class Bio-Weapon Project #154, in a containment cell with a doctor. He kills the doctor and shoots out the ventilation shaft above the room. After fighting his way through numerous guards throught the building, especially in a large elevator, he makes it to the exit where Poseidon is guarding. After defeating him, Project 154 breaks free. Later, he learns of Zeus and Hades pursuing him with the intent to kill him. He then makes it to a small rooftop exit, where a dead Poseidon-like entity lays in a puddle of blood. The entity jumps him, and a "To be Continued" is then displayed across the screen.
The identity of the entity on the roof is never explained, but it can be assumed that this was Zeus before his design was finalized.
Thing-Thing is a mixture of a 2D platformer crossed with shooter elements, in which you take control of Project 154 as he escapes the Systems Corp. building. This game was somewhat inspired by the Madness Interactive series due to the similar controls and style presented in Madness, but much easier in terms of gameplay as well as mechanics. This game was seen as more of a test for the physics engine that would later be tweaked and used in subsequent Thing-Thing games.
- There is a hidden armory stocked with med kits and Beretta 93R's that cannot be accessed through normal means. To get to the armory, the player must right-click and press "forward" to change the .swf's keyframes. It was supposed to be the seventh screen after the cell escape.