Six months after the events of the second film, humans have captured Selene during "The Purge", a global military crusade to exterminate vampires and lycans. Vampires have waged guerrilla warfare against the government, but the humans have overrun them, forcing the survivors to hide underground and resist the humans on their own.
Over the two years since the death of Deacon Frost, Blade (Wesley Snipes), the Daywalker, a vampire unaffected by sunlight and who chooses to defend humans and fight other vampires, has been trying to find his mentor Whistler (Kris Kristofferson), who survived his suicide attempt in the first crisis. Blade has been sweeping across Russia and eastern Europe, enlisting the aid of a young man, Scud (Norman Reedus), to design him a new line of gadgetry and weaponry. Blade carves his way through a large gang of vampires, sparing one, Rush (Santiago Segura), when he promises to reveal Whistler's location. Blade tells Rush he will be back for him. Blade finds Whistler locked in a tank by a gang of vampires who were keeping him alive to torture. Blade rescues Whistler and brings him back to the lair, ridding the latter's vampirism with a cure developed by Dr. Karen Jenson.
A group of vampires look for the hidden resting place of Drake, also known as Dracula—the genetic "father" of their species. They find him in a Syrian ziggurat tomb, although not before, in a moment of blood lust, he kills one of them. The vampires capitalize on Blade's reputation as a serial killer and frame him for the killing of a familiar. During the ensuing manhunt, the FBI locate Blade's latest hideout, and, during the siege, Abraham Whistler dies. With his mentor gone and surrounded by human authorities, Blade allows himself to be captured and arrested.
In 1818, Abraham Lincoln (Benjamin Walker) lives in Indiana with his parents, Nancy (Robin McLeavy) and Thomas (Joseph Mawle), who works at a plantation owned by Jack Barts (Marton Csokas). There, Lincoln intervenes when he sees his friend, a young African American boy, William Johnson (Anthony Mackie) being beaten by a slaver. Because of his son's actions, Thomas is fired. That night, Lincoln sees Barts break into his house and attack his mother. Nancy falls ill the following day, and dies shortly afterwards. Thomas tells Lincoln that Barts poisoned her, but asks that he promise not try to avenge her death.
In 1202, an army led by the three vampire elders (Markus, Viktor, and Amelia) arrives at a Lycan-ravaged village. Viktor and Amelia capture their target: Markus' brother, William Corvinus, the first and most powerful werewolf. Despite Markus' defiance, Viktor orders that William be imprisoned in a secret location forever.
For generations, a secret war has been waged between vampires and Lycans, an ancient species of werewolf. Selene, a Death Dealer, a vampire specialized in assassinating Lycans, tracks two Lycans. Selene's motivation goes beyond duty; she also wants revenge because, 600 years ago when she was a child, Lycans slaughtered her family. The vampires believe they defeated the werewolves many centuries ago and killed their leader, Lucian, and that they must kill the surviving Lycans. Selene locates the Lycans' den and learns they have developed a new kind of bullet capable of killing the vampires.
New York City teenager Clary Fray begins seeing a strange symbol, worrying her mother Jocelyn Fray and her mother's friend Luke Garroway. Later, at a nightclub with her friend, Simon Lewis, Clary is the only person who sees Jace Wayland killing a man. Meanwhile, Jocelyn is abducted by two men, Emil Pangborn and Samuel Blackwell, but she is able to call Clary and warn her about someone named Valentine. Jocelyn drinks a potion putting her in a comatose state. Returning home, Clary finds her mother missing and is then attacked by a demon. Jace appears and kills it. Jace explains that he and her mother Jocelyn are both Shadowhunters (also called Nephilim), warriors that slay demons and rule over the downworlders. Clary has inherited her powers, including the ability to use runes.
The film tells the story of Lucian, the first werewolf born in human form and the first to be called a Lycan. Viktor, a vampire elder, raises the child, and envisions a race of Lycan slaves that could keep guard of the coven's fortress during the day and be used as laborers by the vampires at night. To this end, Viktor demands from the local human nobles a tribute of silver, with which to keep his slaves under control. As Lucian grows up, he and Viktor's daughter Sonja (Rhona Mitra) fall in love and begin a secretive intimate relationship in their adult years. Sonja is reckless and insubordinate, and one night Lucian escapes and rescues Sonja from his werewolf brethren. Viktor, despite acknowledging that Lucian saved his daughter, cannot forgive the escape and locks Lucian away.
Après s'être éloigné de sa fille Ève pour éviter de la mettre en danger, Selene continue sa fuite et sa recherche de son compagnon, Michael Corvin. Elle refuse malgré tout l'aide de David, l'un de ses alliés. Mais quand elle est contactée par les membres du Conseil des Vampires qui acceptent de pardonner ses actions en échange de son aide, Selene est obligée de retourner parmi les siens pour entraîner des vampires à se battre contre les lycans qui préparent en secret une nouvelle attaque avec leur nouveau chef, Marius.
Becca Crane (Jenn Proske) moves to Sporks to live with her clueless father, Sheriff Frank (Diedrich Bader), after her mother starts an affair with Tiger Woods. Meanwhile, killings have been happening to random people and the number one suspects are the Canadians. It is in fact a group of vampires, who are (apparently) commonly confused with the Black Eyed Peas. Becca is quickly befriended by many students at her new high school, (which is odd, considering the welcome she was given) including Jennifer (Anneliese van der Pol) but she is intrigued by the mysterious and aloof Edward Sullen (Matt Lanter), who perplexes her during their time in the biology class, despite thinking she smells of tuna.
A centuries long war between humans and vampires has devastated the planet's surface and led to a theocracy under an organization called The Church. They constructed giant walled cities to protect mankind and developed a group of elite warriors, the Priests, to turn the tide against the vampires. The majority of the vampires were killed, while the remainder were placed in reservations. With the war over, the Clergy disbanded the Priests. Outside the walled cities, some humans seek out a living, free from the totalitarian control of the Church.
Barrow, Alaska is preparing for its annual "30 Days of Night", a period during the winter when there is a month-long polar night. As the town gets ready, a stranger rows ashore from a large ship. Once ashore, he sabotages the town's communications and transport, destroying all means of communication with, and travel to, the outside world. Barrow's sheriff, Eben Oleson, investigates. Eben learns that his ex-wife, Stella Oleson, missed the last plane and must stay the 30 days. Although they try to avoid one another, when Eben confronts the stranger in the town diner, Stella helps to subdue him and take him to the station house.
In 2009, a plague caused by an infected vampire bat has transformed most of the world's population into vampires and due to this event, control of the world no longer belongs to humans. The vampires are incapable of aging or dying, but are unprotected against sunlight or any ultraviolet light. The entire vampire world is active at night, and various technological and architecture advances are made to adjust the populated areas to vampires' night cycle (such as UV protective cars, underground railways and paths across the cities, and UV warnings).