Plot (Spoiler Alert!) Edit
Thing-Thing follows Gamma-Class Bio-Weapon Project #154, as he awakens in a containment cell. He escapes from the cell by killing a doctor, taking his pistol and shooting out the ceiling vent. He fights his way upwards through the building, killing many guards.
When he arrives on the third floor he is halted by Poseidon, an Alpha-Class Bio-Android and one of the three elite bio-weapons created by the facility. 154 defeats Poseidon and makes his exit into open air. During his escape, he learns of the identity of the faction that held him – Systems Corp. – and two other bio-weapons, Zeus and Hades, were deployed to terminate him.
Thing-Thing 2 begins with 154 wandering into a city. Here he encounters Systems Corp. mercenaries but he is able to escape, fleeing into a desert. The desert is inhabited by the undead – creatures who are later inferred to be individuals reanimated as a part of Systems Corp.'s experiments. Battling his way to an industrial sector, 154 fights and defeats Zeus. 154's default weapon is the Heckler & Koch MK 23 SOCOM.
In Thing-Thing 3, 154 begins in self-imposed exile. He receives an anonymous communication from someone who is aware of his origins. Following instructions, 154 fights his way through Sunset City, a housing development for Systems Corp. employees. 154 meets his contact, who is revealed to be Project 153. 153 was developed as an omniscient mobile armor pilot with heightened senses. He reveals that 154 is a form of advanced infantryman designed to be resistant to hydrostatic shock.
The two clash, resulting in the destruction of 153's mobile armor. He casts 154 into a sub-zero Systems Corp. service tunnel filled with more mercenaries. 154 manages to fight his way back towards the top, only to be cornered by 153 again. 154 has now been trapped in a bio-waste disposal chamber and 153 has revived the failed Hades prototype, Vahl Dreig. Thing-Thing 3 ends with 154 defeating Vahl Dreig but still trapped in the chamber. In this instalment, 154's default weapon is the SV Infinity, a competition-use handgun based on the 1911.
Thing-Thing 4 starts with 154 retrieving Vahl Dreig's power core, using it to destroy a wall of the containment chamber. He escapes into the incinerators, where he fights many trained Systems Corp. soldiers. Having resolved to take revenge against the management of Systems Corp., he storms the building's offices. However, there he is confronted by 153 again, armed with a Bren-10 handgun and a small army. 154 escapes the building by jumping out of a window, with 153 in pursuit in an assault helicopter. 154 commandeers a nearby vehicle and shoots out 153's fuel tank using a Heckler & Koch HK21, destroying the helicopter.
154 goes to the genetic research laboratory in search of Systems Corp.'s CEO. There, he learns of experimental Systems Corp weapon systems including undead and synthetic lifeforms, as well as the other 152 previous bio-weapons. He finds the cell he was created in, now empty and covered with warning tape. Fighting through a heavily-defended quarantine elevator, 154 finally encounters the third elite bio-weapon, Hades. Hades is a dishevelled and meek individual, capable only of weak melee attacks but with great physical endurance.
After his first defeat, Hades transforms into a large, spider-like mechanical being, armed with a slow-firing, but extremely powerful rifle. 154 kills Hades, taking his weapon and storming the final room. There he discovers mulitple screen images of the CEO, who reveals that 154 is but one clone in an army of hundreds. The CEO sends a legion of soldiers to kill 154, who is apparently shot dead, due to the room being filled with a gas that negates 154's regenerative abilities. 154's corpse is recovered and used to refine the genetic splicing process.