Stalker

The "Stalker" (Alexander Kaidanovsky) works in some unclear area in the indefinite future as a guide who leads people through the "Zone", a vicinity in which the normal laws of reality no longer fully apply. The Zone contains a place called the "Room", said to grant the wishes of anyone who steps inside. The area containing the Zone is sealed off by the government and great hazards exist within it. At home with his wife and daughter, the Stalker's wife (Alisa Freindlich) begs him not to go into the Zone but he ignores her pleas. In a rundown bar, the Stalker meets his next clients for a trip into the Zone. The "Writer" (Anatoly Solonitsyn) and the "Professor" (Nikolai Grinko) agree to put their fates into the hands of the Stalker. Their specific names do not come up as they all agree to refer to each other pseudo-anonymously by just their professions.
The three of them evade the military blockade that guards the Zone, attracting gunfire from the guards but all survive. They then ride into the heart of the Zone on a railway work car. The Stalker tells his clients they must do exactly as he says to survive the dangers which lie ahead, and explains the Zone dangers are invisible. The Stalker tests for traps by throwing metal nuts tied to strips of cloth ahead of them. The complicated path that they must take cannot be specifically seen nor heard but can only be sensed.
The Writer feels skeptical of any real danger, but the Professor generally follows the Stalker's advice. As they travel, the three men discuss their reasons for wanting to visit the Room. The Writer expresses his concern of losing his inspiration. His manner appears angry and stressed during the journey. The Professor appears more content, though he carefully insists on keeping a backpack filled with unknown contents close to him. While the Professor's desires are not clear, he reluctantly gives in to countless pleas from the Writer and admits he has hopes of winning a Nobel Prize through scientific analysis of the Zone. The Stalker insists he has no motive beyond the altruistic aim of aiding the desperate. At times, he refers to a previous Stalker named "Porcupine", who had led his brother to his death in the Zone, visited the Room, gained a large sum of money, and then hanged himself, failing to achieve the happy ending implied in rumors about the Zone.
While it seems the Room fulfills all the wishes of a visitor, this creates a serious problem given that these might not be consciously expressed wishes but the true unconscious desires of those that come in. In addition it appears that the Zone itself has a kind of sentience. When the Writer later confronts the Stalker about his knowledge of the Zone and the Room, the Stalker replies that his information came from the now deceased Porcupine. After traveling through tunnels the three reach their destination. They determine that their goal lies inside a decayed and decrepit industrial building. In a small antechamber, a phone begins to ring. The Writer answers and cryptically speaks into the phone, stating "this is not the clinic", before hanging up. The surprised Professor decides to use the phone to ring up a colleague. In the ensuing conversation, he reveals his true intentions behind his whole journey.
The Professor has brought a nuclear device with him, and he intends to destroy the Room for fear it might be used by evil men. The three visitors to the Zone then fight verbally and physically in a larger antechamber, just outside the Room. The fight ends in a tie, all three men feeling exhausted. As they catch their breath, the Writer experiences an epiphany about the Room's true nature. He argues that when Porcupine met his goal, despite the man's conscious motives, the room fulfilled Porcupine's true, secret desire for wealth, instead of bringing back his brother from death. Thus, Porcupine's suicide came about from the resulting guilt. The Writer further reasons the Room is genuinely useless to the ambitious since its ability to look inside those who enter it renders the Room only dangerous to those who seek it for negative reasons. With his earlier fears assuaged, the Professor gives up on his plan of destroying the Room. Instead, he disassembles his bomb and scatters its pieces.
The men rest before the doorway and despite their long and arduous journey, they never enter. Rain begins to fall into the Room through its ruined ceiling, then gradually fades away. The Stalker, the Writer, and the Professor are shown back in the bar, and are met by the Stalker's wife and daughter. A black dog that had followed the three men through the Zone is in the bar with them. When his wife asks where he got the dog, Stalker declares that it just came to him, and he remarks that he felt unable to leave it behind.
Later, when the Stalker's wife tells him that she would like to visit the Room herself, he expresses doubts about the Zone. He states that he fears her dreams will not be fulfilled. As the Stalker sleeps, his wife contemplates their relationship in a monologue delivered directly to the camera. She declares that she knew full-well life with him would be hard, since he would be unreliable and their children would face challenges, but she concludes that she is better off with him despite their many trials. "Monkey", the couple's daughter, sits alone in the kitchen, reciting a love poem by Fyodor Tyutchev.
Monkey holds the large book and lays her head on the table before her. She then appears to use psychokinesis to push three drinking glasses across it, one after the other moving over the table. The final glass falls to the floor, but does not break. A train passes by where the Stalker's family lives, and the entire apartment shakes. As the stark, crashing noises of the train begin to subside, the film ends.[5]

Question: What has the Professor brought with him?
Answer: A nuclear device.