"Resident Evil: Degeneration" is a CGI animated film set in the "Resident Evil" universe, serving as a direct sequel to "Resident Evil 4" and a prelude to "Resident Evil 5." The story takes place in 2005, seven years after the Raccoon City incident, and revolves around a bioterrorist attack at Harvardville Airport. The attack involves the release of the T-virus, leading to a zombie outbreak. Claire Redfield, a member of the TerraSave organization, is present at the airport and becomes embroiled in the chaos. She encounters Leon S. Kennedy, a government agent and former Raccoon City police officer, who is sent to investigate the incident.

The plot intensifies when it is revealed that the attack is orchestrated by Curtis Miller, a former U.S. Special Forces operative. Miller seeks revenge against the pharmaceutical company WilPharma, which he believes is responsible for the death of his family during the Raccoon City disaster. He aims to expose the company's illegal experiments with the G-virus, a more advanced and dangerous strain than the T-virus. Miller's actions escalate as he infects himself with the G-virus, transforming into a monstrous creature.

Leon and Claire work together to contain the outbreak and stop Miller. They are aided by Angela Miller, Curtis's sister, who is a scientist working for WilPharma. Angela provides crucial information about the G-virus and helps them navigate the increasingly dangerous situation. The narrative explores themes of corporate corruption, the consequences of bioterrorism, and the personal toll of the ongoing fight against biological weapons.

The film culminates in a high-stakes confrontation at WilPharma's underground laboratory, where Leon and Claire face off against the mutated Curtis. The resolution involves the destruction of the laboratory and the containment of the outbreak, though the broader implications of WilPharma's actions and the lingering threat of bioterrorism remain unresolved. The story maintains a strong connection to the overarching "Resident Evil" lore, emphasizing the persistent danger posed by viral outbreaks and the moral complexities of those who combat them.