Thefour titles, Knee-Deep in the Dead, Hell on Earth, Infernal Sky, and Endgame, comprise a continuing narrative centering around Space Marine Flynn "Fly" Taggart and a ragtag band of human survivors. All are written from a first person perspective, usually with Flynn Taggart as the narrator. Ever since their publication, some Doom fans have insisted that "Flynn Taggart" is the Doomguy's real name, though id Software has never confirmed it, and in this case, it should be noted that the marine (or separate individuals) might have some other names than "Flynn Taggart", or just not have a name at all (which would mean the player would be the marine in this case).
Flynn "Fly" Taggart: The Doomguy, and the main narrator of the novels. Raised in Hollywood, California and Orlando, Florida by his father, who would often embarrass Fly with his crass behavior and criminal activities. Fly joined the Marines to make something of himself and escape his father's behavior. A good marine, until - as with the storyline of the games- he assaulted a superior officer who has ordered unarmed civilians fired upon. He is a non-practicing Catholic who went to Catholic school, though his religious beliefs are rather broad and he still maintains a healthy belief in God. Somewhat mechanically gifted, though sometimes lets his emotions and sense of duty override rationality.
Arlene Sanders: A female Marine who joined to prove she could be part of the Marines. Generally well respected amongst her squad for her willingness to go along with hazing rituals. She and Fly are best friends though avoid a relationship, as doing so would compromise things. She had a relationship with Wilhelm Dodd which ended when he was turned into a zombie on Deimos. She is quite intelligent and is generally more rational than Fly, though tends to be more emotional. She also tends to show a promiscuous sexual attitude towards men she likes. She was raised Episcopalian in her youth, but became a militant atheist in later years. She has problems with the Mormon religion due to her brother converting at one point in his troubled life. (Eric Harris of Columbine infamy was reported to have named his sawed-off shotgun "Arlene" [1], which confused some more casual fans of the series because the Marines were never actually named in gameplay, nor was there even clear evidence of different genders of Marines.)
Albert Gallatin: A devout Mormon and ex-Marine who left after the killing ate away his conscience. He often looks to the Book of Mormon for guidance. He and Arlene develop a relationship though he resists her temptations. The two are married prior to their leaving for the Fred homeworld though are separated shortly after when Albert is severely wounded and can not make the trip with them. All that is known afterwards is that he recovered and returned to Earth to help humanity with the "Doom Wars" and helped the new Earth government. Albert (or a clone) was put in stasis awaiting the return of Arlene to Earth.
Jill Lovelace: A 14-year-old orphan whose parents were killed in the invasion. She appears to be loyal to the Mormon president and will say what he wants to hear, though she seems more willing to help get revenge for her parents' death than she believes in the religion and will say what people want to hear as a result. She is a skilled computer hacker. She also develops a one-sided crush on Fly. After Fly and Arlene leave Earth, Jill is forced to use poison gas to kill two human traitors, an act which does not sit well with her. What is known afterwards is that Jill survived to fight the "Doom Wars" and helped form the new Earth government and wrote history books concerning the war. When Salt Lake City was rebuilt, a copy of her mind was used as the basis for the city's computer. Jill eventually grew old and died, though her mind lived on in the computer and a clone of her was put into stasis awaiting the return of Fly and Arlene.
The "Freds": The Earth military name for the invasion forces. An evil alien race of "deconstructionists", they plan to invade Earth by genetically altering themselves and slave races to look like creatures from Earth mythology - specifically, the Christian idea of Hell - though as a species unfamiliar with evolution, did not expect Earth to defend themselves in the several hundred years it took to organize the invasion. In their natural state, they are said to have stubby bodies with large heads resembling artichokes covered in leaf-like scales and multiple eyes. Their limbs are thin with fingers like sharp chopsticks. They are a technologically advanced race though seem to steal their technology rather than develop it. While they do naturally reproduce, they generally produce their invasion fleet through cloning vats. They are believed extinct as of the end of the series, those on Earth are all killed in the war and those on the homeworld all killed by the "Newbies" as well.
The Klave: the chief representatives of the alien forces opposing the "Freds". The Klave are a "binary" race, of which they remain in mentally linked pairs. They do not understand the concept of individuality and can not deal with individuals. The two main Klave choose the name "Sears" and "Roebuck" by mentally scanning humans to represent themselves though no specific name is ever assigned to either. Killing one half of a Klave pair will generally cause the other to stop functioning and die as well.
The "Newbies": The second race the Klave and "Freds" encounter that evolves. They were encountered by the "Freds" in the 200-year span of Fly and Arlene's trip to the Fred homeworld. In that time the Freds were wiped out by the Newbies. The Newbies eventually left that world and encountered humans, their evolution speeding up until they changed into microscopic beings that inhabit a host body. Generally mindless, and strict, they reduce their hosts to a state of fear. Strangely, whenever a host has faith in anything - a god or an idea - the Newbies cannot survive in the host anymore. Logically, chemicals produced by the body during certain emotional states may be fatal to the Newbies. All Newbies are mentally linked. It is believed their species was literally "evolved" into extinction when one linked to a sped-up computer simulation of Fly and Arlene was drawn into the program, the enhanced rate of time passage forcing it - and the rest of the species - to rapidly evolve until they vanished. Evidence of this may lie on the fact they never reached Earth as they had planned.
The UAC facilities are described as having been built to go into the moons, which means there are not many open skies as in the games, only ceilings, though sky is seen mostly on Deimos (described as a silvery mist) and the Phobos Anomaly (which disturbs Flynn, as the complex is several stories underground). Green slime pools and exploding barrels remain, as do keycard doors.
Chainsaw: Used in "Knee-Deep in the Dead" by Arlene to mow down a horde of demons in the "Hell Keep" section of Deimos. She finds one again in the "Warrens" section, using it in the same exact fashion.
Shotgun: Also called "scattergun" and "riot gun" and from what it looks like, one of Flynn's favorite pieces of weaponry. He uses two varieties on Phobos and Deimos, including a "fascist pump-action" version, which he tends to dislike. Uses the same shell ammunition.
Rocket Launcher: Used primarily in "Knee-Deep in the Dead". Same design, albeit with a few added features not seen in the game (firing pin, infrared laser sight, etc.). Uses "battery-sized" rockets as ammunition that pack the same kind of explosive potential as its PC game counterpart. It can be loaded with two rockets simultaneously, though only one can be shot at a time. In the books, Flynn discovers that he can use the rockets to destroy doors. Such an occurrence is not possible in the games.
Plasma Gun: Mentioned in "Hell on Earth" and used in "Infernal Sky". Flynn expresses disdain at bringing "one of those gas-powered plasma guns I never really cared for" for the L.A. mission, but it does not appear to have been used. Arlene ends up using a plasma gun on the superpumpkin when she and the other Marines return to Phobos.
BFG9000 : Used extensively at the end of "Knee-Deep in the Dead" and in one part of "Infernal Sky" - Flynn find it in Warrens (nicknaming it the "big freaking gun") and uses it throughout the rest of the book, expended the last of the ammo in the finale. Flynn comes across a BFG on Phobos when he and the other Marines return. The book does not cover the ammunition used for the BFG.
Other weapons not in the game appear in the books: for the L.A. mission, Flynn arms himself with a hunting rifle as a secondary firearm; Arlene's weapon of choice is an "AB-10" machine pistol, and she uses a "scoped .40-40" (a sniper rifle variant) for the L.A. mission; Albert is usually armed with a strange "Uzi-clone" and Arlene's .40-40 for any sniping; Jill first learns to use guns with an "AR-41", an automatic rifle she becomes quite fond of and which never leaves her side. Marines on the Phobos and Deimos installations are typically armed with a 10mm "Sig-Cow" automatic rifle that Flynn usually arms himself with, whether picking it off a corpse or from a zombie, but considers it a last resort weapon (he doesn't appear too fond of it, preferring something with more substantial firepower, but understands that it holds better stopping power than a pistol of the same caliber). The Freds are armed with two different weapons: a strange gun that fires out small, spinning UFO-like discs, and the "Fred Ray", a smaller handgun that fires an invisible, disintegrating ray.
Berserk Pack: Seen once. Arlene comes across one in the Deimos refinery in "Knee-Deep in the Dead", claiming that the pack contains a powerful dose of adrenaline best used for people in the throes of cardiac arrest. Using it on a healthy person will up their adrenaline amount to the point of madness-inducing, but at the risk of tachycardia, leading to death. Flynn decides to use it during an imp ambush, resulting in a temporary psychotic rage with the same effect as in the game taking place (red vision, increased strength). However, instead of his fists, Flynn opts for his bayonet to cut the imps apart. He doesn't contract tachycardia, but suffers a serious bout of fatigue as an after effect.
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