The story revolves around a confrontation between two opposing supernatural groups (known as "Others"): the Night Watch, an organization dedicated to policing the actions of the Dark Others—and the Day Watch, which polices the actions of the Light Others.
In the story's worldline there exists a magical realm beneath the surface of all things—referred to as the Twilight (or Gloom in other translations). The action in the novel centers around a group of people referred to as the Others—human beings who tapped into the Twilight and gained supernormal abilities. The Others were the humans (shamans, soothsayers, and wisemen) from long ago who figured out how to step into the Twilight. However, the Others are different from humans, they are born as Others. Humans are not able (at least in the first part of the tetralogy) to become Others. The Twilight does not offer its gifts freely; it feeds off the strength of those Others who enter it. If sufficiently weakened, they are consumed, never to return to the ordinary world. The aura of any Other, or emotional state at the time of entry into the Twilight, determines whether or not the an Other will become a 'Light' or 'Dark' Other. Furthermore, once determining Light or Dark, an Other must choose what specific powers they will borrow from the Twilight. Variations such as vampires, magicians, healers are all possible, each with their own benefits and restrictions. Often, the choice is made by the state of mind but if choosing Light or Dark during Initiation, a Watch can attempt to steer the powers of someone into what they need at the time. "Initiation" refers to the process of an other choosing not Light or Dark but choosing to officially be a part of the Night Watch (Light Others) or Day Watch (Dark Others). An Other can exist without being initiated as part of a Watch, still independently capable of entering the Twilight and becoming Light or Dark. The choice of becoming light or dark, even what specific powers you gain is usually final.
The division of Light and Dark had always existed between the Others. Those of the Light believed it was their duty to help the weak and the helpless. Those of the Dark shunned all obligations. They did what they wanted, regardless of morals and consequences. For many millennia, the two sides fought a vicious battle. Both were willing to use any means necessary to achieve victory. Eventually they realized that if they continued their battle, neither side would survive. The leaders of both sides forged the Grand Treaty—a set of laws to govern the way the Others used their powers. The Light Others created the Night Watch, the Dark Others the Day Watch, to ensure that neither side would violate the Treaty. The Inquisition, a group composed of both Dark and Light Others, was created to ensure that neither side became too powerful.
If they spend them too quickly, the Others can use the feelings and emotions of the humans surrounding them to recharge their powers. The Dark Others use negative emotions such as pain or anger, the Light Others use positive emotions such as joy. Feeding on pain causes pain to increase, feeding on joy causes joy to wane. Because negative emotions are much easier to achieve in humans, this arrangement creates a situation where the powers of the Dark Others are easier to recharge and are much more readily available than those of the Light Others.
Since the signing of the Treaty, the Night Watch and the Day Watch have kept their eyes on each other, diligently policing every violation. The old leaders continue to plot, using humanity and the Others as their pawns. Only time will tell which side will prevail.
The book comprises three stories, each one seemingly independent at the first glance, but they are in fact connected.
A reluctant mage recently reassigned to field work, Anton Gorodetsky tracks vampires by drinking blood and channeling them. As he follows a young boy (Egor), who has been called by two vampires through the Metro, he notices a young woman, Svetlana, who has a huge vortex of damnation above her. Anton finds the vampires who have been calling Egor with their power and, as they don't have a special license, kills one of them, while the other (a female) gets away.
He returns to the Night Watch headquarters, where his boss, Boris Ignatievich, informs him that he could be in danger as Zabulon (head of the Day Watch) might want revenge for his actions in killing Dark Others and gives him a stuffed owl called Olga (who later transforms into a woman), for his protection. Anton initially rejects the offer, then finds Olga in his apartment and reluctantly agrees. The next day he illegally uses his powers for good (by changing a person's morality, a spell called "remoralization") and clashes with a Dark Other from the Day Watch, Alisa Donnikova. They agree that Alisa can use her power to do a minor evil act as an exchange for her silence about this matter. He then discovers that Olga can speak and is a sorceress trapped in an owl's body as a punishment.
Anton goes to Egor's apartment to protect him from the vampire, then nearly loses him as Egor inadvertently slips in the Twilight for the first time. Surprisingly, he remains unaffiliated both to Light or to Dark. Meanwhile, Boris Ignatievich sends an incubus, Ignat, to the cursed woman, Svetlana Nazarova, to help her relax and say who could have cursed her, but to no avail. Finally, Anton is reassigned to visit Svetlana, and in the meantime, the vampiress calls to Egor and he joins her on the roof.
Anton manages to find out that Svetlana is a powerful Other who inadvertently cursed herself with her guilt (for what she did to her mother). Anton then gets back to Egor's roof to find the vampiress and Egor, as well as many other Light and Dark operatives, including Anton's neighbour, Kostya Saushkin, who is a vampire. Zabulon joins them and attacks Anton. But when he does, one of the Light Mages, Ilya, reveals himself to be Boris Ignatievich, who'd swapped their bodies. While Zabulon's plans seem compromised, and Egor considering his choice for Light, in a last shot, Alisa uses her agreement with Anton to make him tell Egor everything. Anton reveals to Egor that he was a pawn used by Boris Ignatievich and Zabulon in their intrigues. Angered, Egor leaves the roof, feeling it was better for him to not to stay with any of the forces.
A Dark Other, Galina Rogova, is killed by Maxim, a mysterious murderer using an enchanted wooden dagger, referring to himself as "The Judge". The next day, Boris Ignatievich announces that the Day Watch suspects one of the Night Watch operatives. Everyone seems to have an alibi, except Anton. Boris Ignatievich thinks that Anton has been set up by the Day Watch, and uses his powers to swap Anton and Olga's bodies (Olga being now permanently human). In Olga's body, Anton then goes to Svetlana's to hide, and uneasily reveals that he is in fact Anton, making Svetlana furious as she just told "Olga" she loved Anton. They go to a restaurant later in the evening where they spot an inoffensive Dark Other with his family. As the Dark Other goes to the toilet, Maxim follows him and kills him. Unfortunately, it's Anton who discovers the body. Boris Ignatievich then reveals himelf and asks Svetlana to give him anything that could help to recognize the killer, and Svetlana telepathically sends him the image of Maxim's wife's aura. Zabulon then comes over and recognises Anton in Olga's body. Anton flees while Zabulon tries to hit him with an effective destructive power, and manages to hitch a car. What he doesn't know is that the couple in the car are Maxim and his wife. Walking home, Anton calls Olga and asks to switch bodies back.
Waiting in the subway for Olga, Anton stumbles upon Egor and has an unpleasant conversation with him. Olga arrives and Anton and Olga switch their bodies back using an incantation that reveals Boris Ignatievich's real name as Gesar. While talking to Olga, Anton realises that the Day Watch is only chasing him in order to make Svetlana mad and use her powers illegally, which would allow them to dispose of her, or at least incapacitate her. Anton then takes a ride in the metro and when he goes off at a station, he finds himself in front of the Ostankino Tower. A gloomy figure, a departed mage, indicates that Anton should go to the Tower. There he meets a Dark mage, who makes him understand that the Day Watch has established their temporary headquarters in the Tower. He kills the mage and uses his appearance to mask himself. He arrives at the headquarters to discover Tiger Cub as an inspector from the Light and a bunch of incompetent Dark mages directing his search. Leaving the Tower, Anton goes by Egor's house and thinks Zabulon is taking revenge on him by setting him up.
Meanwhile, Maxim feels the presence of a Dark being and goes on a hunt. He finally finds the Dark being and is astonished when he discovers the it is a little boy, Egor. Anton spots them and talks with Maxim, explaining the Treaty between Light and Dark, but Maxim doesn't comply. Gesar then comes over, suggests that Maxim should become a member of the Inquisition, and when Anton brags about how he outwitted Zabulon again, Gesar reveals that Zabulon has nothing to do with it and that all that was planned by the Night Watch to raise Svetlana's magical level.
All of the Night Watch operatives go to Tiger Cub's house to relax, but Anton doesn't manage to have fun, as he is concerned with Svetlana's growing powers influencing their relationship and the reason Geser sent them off. He finally leaves and when he comes back to his apartment, he discovers Zabulon calmly reading a newspaper and waiting for him. Zabulon reveals that Alisher, the young man from Uzbekistan, brought with him an artifact called the Chalk of Fate and that it could be used by the Light to establish a new world order by rewriting a destiny of someone who could later change the world. Discussions with Olga and later Geser reveal that Svetlana in fact must use the Chalk to rewrite a destiny.
Walking outside, Anton drains the Light power from all of the passers-by he sees, taking their joy away. Anton joins Gesar, Svetlana, Zabulon, Egor, and Maxim on a rooftop where Svetlana prepares to rewrite a destiny, while a storm is gathering around them. Svetlana then opens a representation of Egor's fate. Gesar supposes Anton could use all the energy he has drained to stop the storm, but Anton uses it instead on himself via a simple remoralization spell. Astonished, Svetlana stops rewriting Egor's destiny and asks Anton for advice, but Anton says that she must decide what to write herself. The Book disappears, and Gesar notices she didn't write anything, she only erased things. Egor reverts from a Dark Other to an unaffiliated state. Zabulon notices that their planned operation failed because of Svetlana's indecisiveness and, triumphant, leaves. Anton then notices the Chalk Svetlana used is not whole. Further discussion between the Light operatives reveals that Svetlana rewriting Egor's destiny was just a distraction, and in the mean time, Olga rewrote the destiny of someone later revealed. Gesar reveals the true nature of this plan was to save his love of Olga. Without her full powers, their love was doomed.
The Twilight, although a realm of existence beneath the human or ordinary world and therefore not sentient, indiscriminately absorbs energy from whomever enters it. Whenever an Other enters the Twilight it starts to drain his or her strength. The only way to survive in the Twilight is to slowly feed it energy. The skill required to stay (or even enter) into the Twilight for an extended period of time is acquired through formal training. Others who lack this skill yet still manage to enter the Twilight are at an increased risk of being completely drained of their life and energy.
There are stories of magicians who have been lost to the Twilight either because they exhausted their energy in battle (within the Twilight), or because they were careless. Novices have also been lost to the Twilight because they have allowed it to take too much energy; they lack the skill to "feed" the Twilight.
The length of time that an Other can stay in the Twilight depends on their level of power and their level of skill. However, even magicians who are quite powerful, adept at entering the Twilight and feeding it, cannot stay in there indefinitely.
"Regular" laws of physics, that is, the laws of physics present in the ordinary (human) world do not apply in the Twilight.
Others have colour in the Twilight as do their emotions and the magic that they manifest. Similar to reading a person's aura, the colors present in or around an Other in the Twilight can be read in order to gauge their feelings.
The Twilight is what imbues the Others with most of their magical powers. It is also responsible for giving them their unnaturally (in human terms) long life. Due to the Twilight, Others can live to be hundreds, possibly even thousands, of years old.
The Others have a twilight form that they can take on in or out of the Twilight. Most Dark Others take on the form of a demon like figure. Zabulon takes on the form of a red rock like skinned demon with horns and a tail,also with slits for eyes and a forked tongue.
The Twilight itself is divided into several levels. Each level is harder to get to and accessible only to powerful Others; this is due in large part to the fact that the Twilight drains energy much more quickly and greedily in deeper levels. In order to access the lower levels of the Twilight, an Other must step through their shadow at each successive level.
At the first level the Twilight is a gray or sepia-toned version of the human world where time runs much more slowly than in the human world; this means that you can do more in the Twilight in less time than it would take in the human world. In this level remnants of the human world still exist and can be seen. For instance, humans can be seen but they are sepia-toned and are moving at a much slower rate due to the time difference between the two realms. This level is inhabited by a parasite, a blue moss feeding on energy, which doesn't seem to do any harm to those who enter the Twilight, and is easy to eradicate. At the contrary of what we see in the movie, there are no mosquitoes in this level of Gloom.
The second level is even less like the human world—all human buildings and people disappear, but all magically built building change into a different version of itself. It is described as being darker, and having a consistency that is comparable to clouds.
The third level is similar to the second, the only major difference being that the third level is darker and drains energy more quickly.
The fourth level is the level where the darkness begins to recede and colors start to appear. It is much harder to access this level. Unlike the first three levels of the Twilight, this level is more lifelike.
The fifth level is almost habitable. In this level clouds, lightning and grass appear. The colors are much brighter than in the fourth, and it is more similar to the human world.
The sixth level is almost the same as the human world. All the things in this level are "almost real", as described by Anton Gorodetsky in "Final Watch". It is a place which has everything that is in the human world, except for life. Every Other that dies goes to this level, but occasionally some Others that are powerful enough rise to higher levels of the Twilight to help Others (as in "Night Watch" when Merlin rose to help Gorodetsky). In the higher levels of the Twilight, these Others appear as blurry, grey human figures made of fog.
The seventh level is unknown, as no one has ever made it there (or so the Others are told); however, the mystery of the seventh level is finally revealed in the fourth book (Final Watch).
The Twilight is accessible only to Others (both living and dead), particular magical objects, and certain animals. Others access the Twilight by raising their shadow, increasing its volume and density, then stepping through it.
As mentioned before, particular magical objects can enter the Twilight. However, it is not so much that they can enter the Twilight as that they exist on multiple levels of reality. For example, a copper amulet in the human world may be seen existing in the Twilight as a small ball of fire.
Night Watch hinted that animals do not always follow the same rules as humans or Others when it comes to the Twilight. In the first book of Night Watch entitled Story One: Destiny, Anton explains that "For cats there is no [human] world or Twilight—they live in all the worlds at once."
There is only one natural inhabitant in the Twilight, a dark blue moss that grows along many surfaces of the sepia-toned human world in the first level of the Twilight. It feeds off of the emotional energy of Others who enter into the Twilight and of humans in the ordinary world. There is a theory mentioned in the books that states that large colonies of dark blue moss can unbalance the human psyche and cause psychosis but it has not been proven. Nonetheless, the protagonist of the Night Watch—Anton—occasionally feels compelled to utilize magic to burn off the blue moss at points in the story.
The Twilight is still largely an unknown. This may be due to the fact that there are very few truly powerful magicians who can successfully navigate its levels. Night Watch mentions that the deepest levels have been navigated by Zabulon (see character descriptions below) but at the cost of great energy expenditure even to him.
When an Other dies of natural causes, is killed, or dematerialised (executed) their "soul" or "essence" is passed into the Twilight where it remains for all eternity. Since Others are 'born' in the Twilight it would make sense for them to return there upon their death but the reason for this is never stated in Night Watch. They become spectral-like figures, cloudy silhouettes of their former physical selves. It remains a mystery whether or not they retain any memory or feeling as they hardly communicate with living Others who enter the Twilight.
In the second part of the Night Watch, what is assumed to be a dematerialised Other in the Twilight seemingly communicates with Anton as it points him towards the TV tower where the Day Watch has set up its field headquarters as it searches for Anton (who is being framed for killing Dark Others). Anton is unsure if it is a dematerialised Other, or if it is simply a part of a trap put into place by Zabulon.