Set one year after the events of Biohazard 4, Claire Redfield is now a member of a non-governmental organization which handles search and rescue of Chemical and Bioterrorist attacks known as Terra Save. Claire arrives at Harvardville Airport to meet with a friend and her niece Rani Chawla. While waiting, she strikes up conversation with a blond Englishman over the news before he decides he's waited for whoever he's waiting on long enough.
Meanwhile, a passenger jet is approaching the airport for landing. As one of the attendants checks the passengers, she notices one man is looking unwell. The man in question is clearly in the final stages of the deadly t-Virus infection and requests that she reach a WilPharma contact for him.
Back at the terminal, Senator Ron Davis is attempting to circumvent the myriad of human rights demonstrators surrounding the airport, protesting his involvement with and support of WilPharma, and their alleged research into the t-virus.
Rani recognizes Davis and points to him, drawing the attention of a nearby television reporter. She attempts to interview him when they are interrupted by a protester wearing a zombie mask. Davis' bodyguards and airport security quickly apprehend and unmask the demonstrator who is taken under arrest. When this happens Claire tries to help but is almost taken in as well. Just then an actual zombie appears. The zombie quickly kills both the airport security guard and one of the Senator's bodyguards when suddenly an entire pack of zombies swarms the terminal, killing several more people, including the reporter and the senator's remaining bodyguard.
As Claire rescues Davis, the jet suddenly crashes into the terminal, sending debris and shrapnel everywhere. To Claire's horror, the plane's door bangs open revealing all of its occupants are now also zombies.
Claire, Davis, Rani and an unnamed woman take refuge in the terminal's VIP lounge as day turns into night. The Marines have locked down the airport, and established a perimeter between it and the majority of the survivors, who are quarantined and being evacuated for treatment one at a time.
Two Special Response Team (S.R.T.) members, Angela Miller and Greg Glenn discuss the situation in their tent when Leon S. Kennedy a survivor of Raccoon City, enters and introduces himself as the expert government agent that will be leading their task force. He orders Angela and Greg to accompany him (citing that he only wants two partners, as any more would increase the risk of having to battle infected teammates), and the trio enter via the roof of the building (en route Leon explains how zombies can only be killed by shooting them in the head).
Angela and Greg almost immediately disregard Leon's instructions and are both almost killed by zombies before Leon saves them and demonstrates the head shot technique.
Claire, upon hearing a survivor nearby, sets off to explore, armed only with a red and white umbrella (an irony she comments on). Eventually she runs into the S.R.T., and the now larger group decides to flee the terminal by sprinting through the lobby. As they run through, Greg is bitten by a zombie while trying to carry the injured Davis and opts to stay and fight off the encroaching horde to buy the others more time.
The group makes a stand on top of the crashed jet, and Davis makes a dash for the exit, knocking Rani off the plane in the process. Both Claire and Leon are almost killed by zombies trying to rescue her. Both Davis and the rest of the group manage to escape into the arms of the waiting military, but not before Angela looks back and sees a now undead Greg.
While the Marines quickly move in and begin containing the inner terminal, Claire recognizes WilPharma trucks arriving on scene and demands to know why. The Senator and the Englishman she was talking to, revealed to be named Frederic Downing, reveal that WilPharma is actually working on a vaccine for the virus. In fact, all of the Marines employed on this mission are vaccinated against it. This news infuriates Angela, as a vaccination would have saved Greg's life. Claire too is distraught after she realizes her organization has been bullying a company with noble intentions and only made things worse.
Suddenly, the WilPharma trucks containing the vaccine explode. The group quickly deduces that this may be yet another bio-terrorist attack and Claire and Frederic rush to WilPharma headquarters to check on the raw vaccine data. Claire having recognized disgruntled WilPharma scientist Curtis Miller at the airport just prior to the zombie attack, Leon and Angela (revealed to be Curtis' sister) decide to check out Curtis' home, suspecting he may be responsible for all the recent bio-terrorism attacks.
Frederic gives Claire a tour of the WilPharma facility (a quartet of laboratories circling a large shaft with an even larger basement area below it) before confiding in her that Senator Davis requested that he not show her what he's about to—that the company also has samples of the G-virus, which causes Claire to flash back to her dealings with William Birkin. She insists on telling Leon but Frederic begs her not to for his sake before noticing that the server has gone down. He excuses himself to fix it.
Angela and Leon arrive to find Curtis' house in flames (only a charred picture of his family and Angela is saved). Angela reveals that Curtis lost his family in the Raccoon City incident. Claire calls Leon to inform him of WilPharma's possession of the G-virus, she then answers a call coming from within the building which turns out to be Frederic. He warns Claire of a time-bomb in level 4, She then hears a bell sound in the line. The line goes dead and Claire recognizes Curtis Miller fleeing with a briefcase before the bomb goes off as she is partially caught in the blast of the ensuing explosion (a piece of glass puncturing her calf in the process which greatly hindered her movement capabilities).
Leon and Angela arrive at the building, Leon goes upstairs to Claire whilst Angela monitors the ground level. Leon directs Claire to the escape elevator, which leads her to the central control room. Meanwhile Angela finds Curtis in the remains of the air garden (the central shaft) and discovers that he has infected himself with the G-virus. He angrily claims that he's done what he has because the government covered up some of the events in Raccoon City, which kept his family from being saved.
The Marines suddenly storm the room with guns drawn. Curtis begins to mutate and, despite taking an extended barrage of automatic weapons fire, begins to slaughter them with ease. As Curtis' attention falls on Angela, Leon enters and uses a grenade launcher to bring a large chunk of scaffolding down on Curtis, crushing him.
The building's security announces that the entire room will be sterilize via incineration and the pair plus the surviving marines scramble for the emergency elevator. The marines fill the elevator cab and Leon and Angela agree to take the next lift. Unfortunately, Curtis mutates into an even more monstrous creature and chases the elevator up the shaft before ripping it free and hurling it and the marines to their death.
Leon realizes that Curtis now sees Angela as a breeding tool due to their shared DNA. The two dive into a nearby pool of water to escape the incineration, and Leon shoots the water tank open, dumping them into the center of the quartet of underground labs. They quickly realize that Curtis has also survived the blast as he quickly begins stalking them again.
Despite Claire's efforts, the building detects Curtis and initiates a biohazard lock-down, which involves a sequence of releasing each of the labs in a free fall into the cavernous basement below. Curtis manages to separate Leon and Angela before he finally recognizes her through his now degenerated brain. He tells her to escape but she decides to stay, saying that it's time to finish things. However, Leon grabs her from behind while standing on a scaffold as the once again devolved Curtis, and the last lab plummet into the abyss.
Curtis manages to snag Angela's ankle with his tail, and though she begs Leon to let her go, he refuses, repeating Curtis' motto Angela adopted ("If you don't try to save one life, you'll never save any"), and shoots Curtis in the head, causing him to let go and fall to his death.
Leon, Angela and Claire exit the building and Claire accuses the just arrived Senator Davis of being behind all of this in order to drive up WilPharma's plunging stocks (Davis is a major stockholder). Davis denies it, and Leon agrees that Davis is not responsible. It soon dawns on Claire, who had found a recording of everything involving the mutated Curtis in the control room and heard a bell sound like she heard when she talked to Frederic, that Frederic is actually the one responsible, and she quickly finds the payphone that he ripped out to feign his emergency call earlier.
On a road overlooking the city, Frederic gloats over the phone with General Miguel Grandé, revealing that thanks to him, Grande is now the only man in possession of both the t-virus, the t-vaccine, and data on the G-Virus, though he warns the general about the G-virus, describing it as "wickedly volatile." A car pulls up, and Frederic believes it to be his contact. However, it is quickly apparent that it is Leon and Claire. Angela scares Frederic by holding a gun to his head as he pleads to make a deal with them splitting the profits, before revealing that the gun isn't loaded as a bevy of marines and police swoop in to arrest Frederic.
The next day, Leon and Claire meet with a despondent Angela. Leon reveals that Frederic has confessed to everything, including all of the bio-terrorist attacks and the airport incident, all of which was designed as a type of sales pitch for the virus to General Grandé. He reveals that Frederic was an Umbrella researcher who stole samples of both viruses and escaped immediately prior to the Raccoon City incident before erasing his history and creating a new identity with which to apply to WilPharma. Once there, he was able to shop out the virus to a list of potential customers while researching the vaccine. Leon and Claire then walk away from Angela, and Leon offers Claire a ride in his helicopter, and she responds by saying that she already has a limousine waiting for her. Leon then tells Claire that he hopes that next time they bump into each other, it will be some place a little more normal, to which she agrees. Claire then watches as Leon boards the chopper and flies away, before heading towards Rani and her aunt.
Later, in Senator Davis' office, the news goes on about how he has resigned from office over inside trading allegations involving WilPharma. Davis sits in his office chair with a newspaper with the headline "TRICELL OFFERS TO BUY WILPHARMA" over his face. Suddenly, his cold hand slumps to his side and it becomes obvious that he is dead. As his body sits there, his computer continues to purge all information regarding the viruses and pharmaceutical companies (how he died and who initiated the purge is not revealed).
Later people in biohazard uniforms with the TRICELL logo on it continue an exploration of the basement level of the WilPharma compound. One of them discovers the severed tip of Curtis' mutant tail and begins sealing it in a biohazard container.