Bit of a stretch trying to recap this series but eh we'll give it a shot.The games go in this order.
House of the Dead Overkill: Set in 1991, a mutant outbreak spreads through most of the Louisana swamps. Rookie AMS agent, G, is dispatched to investigate the matter partnered with Issac Washington, a local and rather foul mouthed detective, who is after Papa Caesar the suppose instigator of the outbreak and the murderer of Issac's father. Also on his trail is Varla Gunz who likewise wants revenge for the death of her brother Jasper who was forced into helping Caesar to keep Varla safe. Issac and G blast their way through the area (while likewise Varla has her own adventures with Jasper's stripper girlfriend Candy which ultimately ends in the latter's death), all parties convene at the local prison where it's revealed the warden, Clement, is the true mastermind having started the outbreak as a side effect in curing his ailing mother. He kills Caesar (to Issac's dismay) and kidnaps Varla to be the new body for his mother. Issac and G fight their way to him but it's too late, he's completed the operation, put Varla's brain in a jar and his mother's brain into Varla's body which somehow grants her psychic abilities. However the procedure backfires and Clement mother mutates to a giant grotesque monstrosity. Issac and G defeat her and Clement makes amends by "going back into her womb". The dilemma over, G takes Varla's brain and the two ride off, but not before a tape, that was given to Issac from Caesar, is play in which he explains that Clement had "powerful friends" backing him so things are far from over. He also mentions that Issac's father is still alive.
Interlude: One of said "powerful friends" turns out to be a scientist name Curien who wishes to find a cure for his ailing son Daniel. With no answer in medical science, he started looking into research of life and death, back by a company called the DRB Corporation. However, the further he went into his research the more deranged he became until eventually...
The House of the Dead: Seven years later, G and his partner Thomas Rogan receive a distress call from their friend Sophie who was one of the researchers in Curien lab which coincidentally was also his home. The two agents arrive to see researchers being slaughtered by zombies, the results of the experiments. The two agents fight their way in and managed to find Sophie, but she's mortally wounded by the Chariot, one of Curien's enforcers. Before she passes out, Sophie begs the two to stop Curien. G and Rogan honor this request and make their way through the estate fighting all manner of monsters and saving any trapped researchers they can. Eventually they reach the heart of the lab where Curien awaits. He activates his ultimate creation, the Magician, to kill the two. But the Magician turns on him first and kills him. G and Rogan battle the monster and manage to destroy it. But the Magician leaves a warning that they haven't seen anything yet. With that, the two take their leave of the mansion.
The House of the Dead 2: A year after the events of the first game, another zombie outbreak occurs in Venice Italy. G is sent to investigate what's going on but contact is lost with him. Thus two more agents, James and Gary, head out to find him. Almost instantly they're attacked by a winged creature who warns them to leave. Disregarding it, they make their way to the proposed meeting spot and find G mortally wounded. Giving the two a journal of what he's found so far, he pleas with the two to save the city. The two oblige fighting off the monster while saving any citizens that they can. Along the way they're joined by Amy and Harry, two more AMS agents sent to back them up. The four continue onward till Harry and Amy are kidnapped by an unknown foe. Jame and Gary head for the collisum to save the pair where the mastermind behind the outbreak, Goldman, taunts them. After defeating the monster sent after them, James and Gary proceed alone to the DBR Corperation where a revived Magician fights them. After defeating it, they proceed up the building where Goldman await. He claims he's doing all of this as he believe humanity is destroying nature and must be punished. He then unveils his latest creation, the Emperor, to fight the agents. They eventually manage to defeat it. But Goldman refuses to be taken alive and jumps off the building, but not before warning that a successor will take his place. With the ordeal over, the two regroup with their fellow agents and leave the city.
House of the Dead 4: Three years later, James still can't quite get the events of the second game out of his mind feeling they may have missed something. He heads back to Italy with his new partner, Kate Green, to look over files on the case in the AMS European branch. However as they reach the basement floor where they're kept. An earthquake rocks the building and traps them in the room. After waiting several days for rescue, the two notice on a monitor that zombies are coming for them. Arming themselves with machine guns, the pair fight their out and through the building, stopping at a computer room to find out what's happening and realizing that nuclear missiles are set to go off within 24 hours unless they reach the DBR Building and stop it. Fighting their way topside they see that the city has been once more devastated by the outbreak. Though disheartened a bit, they carry on and eventually reach Goldman's office where they stop the missiles. However Goldman, in a message he recorded before his death, explains that he had a backup plan in case either the Emperor or the missile plan didn't work. Another monster he calls "The World" of which his computer had continued to work on after his passing. The missiles deactivation trigger it's activation. James and Kate combat it but it continues to evolve after extensive amounts of damage. In a desperate move, James set his PDA to explode and rushes the creature, his parting words to Kate on not to give up. His sacrifice succeeds and The World is destroyed. Kate morns for him unsure of what to do, until she meets G outside the building who requests her help. As they head off, a final recording from Goldman states to head North as they're might be survivors there.
House of the Dead 4 Special: Following the end of 4, G states he found the source of where the zombies are coming from and Kate and he proceed into a factory blasting many monsters along the way. Eventually they reach the center where they find the cause to be coming from the Magician, once more resurrected. Another lengthy fight ensure to which, near defeated, the Magician rushes for a button that will unleash clones of him. However a well tosses grenade from G prevents this and destroys the creature once more. As G and Kate leave, a mysterious man with a limp appears, contemplating about how Pandora's Box was closed but that they'll never stop the wheel of fate.
House of the Dead 3: In the year 2019 (16 years after the 4th game), the zombie outbreak has devastated the world and AMS long shut down. However one ex-agent, Rogan thinks he's found where it may be coming from, an old ERI research facility. He and a team of commanders raid the building and indeed find monsters inside. However near all the team are killed save for Rogan and one other member. The two reach the heart of the facility and the source. But before they can do anything, the remaining commando is killed and Rogan is knocked out, the last thing he sees is a pair of footsteps coming his way.
Two week later, Rogan's daughter, Lisa and his former partner, G, track his whereabouts to the facility with the hopes of finding him. Armed with shotguns they make their way through the building fighting off the zombies and monsters within. Eventually they reach the same area where Rogan's group reached and find him alive and well. As well as another person, Daniel, who had saved him. He explains that Curien did manage to cure him of his illness but by then he was too obsessed with his research to stop. As he finished the Magician in the first game, Curien began work on a second monster known as The Wheel of Fate which is in fact Curien resurrected in the event something happened to him. Daniel, wanting to put an end to Curien's legacy, wishes to stop it which Lisa volunteers to help him while G helps Rogan out of the building. Lisa and Daniel fight their way to where the Wheel of Fate is just as it begins it's activation. A tough battle ensure but the two come out victorious. The two regroup with G and Rogan and leave together, but not before Daniel swears if humanity goes astray again he will return to the building. Unbeknownst to the group as they leave, the man with a limp appears in the main lab and grabs the vial that cured Daniel stating that Curien "didn't understand it's true purpose" and chuckles evilly to himself.