A really short synopsis of Frankenstein, or The Modern Prometheus would be:

A young Swiss student discovers the secret of animating lifeless matter and, by assembling body parts, creates a monster who vows revenge on his creator after being rejected from society.

A more detailed summary would go like this:

An English explorer, Robert Walton, is on an expedition to the North Pole. In letters to his sister Margaret Saville, he keeps his family informed of his situation and tells about the difficult conditions on the ship. One day when the ship is completely surrounded by ice, a man in bad condition is taken aboard: Victor Frankenstein. As soon as his health allows it, he tells Walton the story of his life.

He grew up in Geneva, Switzerland as the eldest son of a higher class family. He was brought up with an orphan, Elizabeth and also had two younger brothers. He did not have many friends, Henry Clerval being the only exception. At the age of nineteen, Frankenstein became interested in natural philosophy, electricity, chemistry and mathematics. After the death of his mother, who succumbed to scarlet fever, Frankenstein left for Ingolstadt, Germany, to attend university. There, his interest in natural philosophy quickly became an obsession. He was particularly fascinated with the human frame and the principle of life. After four years of fanatic studying, not keeping in contact with his family, he was able to "bestow animation upon lifeless matter" and created a monster of gigantic proportion from assembled body parts taken from graveyards, slaughterhouses and dissecting rooms. As soon as the creature opened his eyes, however, the beauty of Frankenstein's dream vanished: it became a horrible creature. He realised he made a mistake in creating this monster and fled from his laboratory. On his return the next day, the monster had disappeared. Victor was consequently bedridden with a nervous fever for the next months, being nursed back to health by his friend Clerval. On the eve of the return to his parental home, he received a letter that his youngest brother had been found murdered. On his way home, Frankenstein saw the dæmon he has created and immediately realised that it is he who is responsible for his brothers death. Frankenstein decided not to tell his family about the dæmon because they would simply dismiss it as insane. As he arrived home, he was informed that the murderer of his brother had been found. The accused was Justine, a good friend of the family. When Justine has been found guilty and has been hanged, Frankenstein's heart was tortured. He could not stay in the house and started wandering in the Alpine valleys. There, Frankenstein was confronted with his creation who tells him his life story.

After leaving Frankenstein's laboratory, he went to the village where he was insulted and attacked by the frightened villagers. He eventually went to the country and found refuge in a hovel next to small house inhabited by a old, blind man and his two children. By observing the family and by reading their books, the monster learnt how to speak and read. He felt compassion for the family who have to struggle to get by, and anonymously did chores for them. Longing for some kindness and protection, he decided to meet his hosts. He got into a pleasant conversation with the blind man but his children return unexpectedly. Horrified by his appearance, they beat him and he fled the house. Completely disillusioned, the monster was filled with rage and decided to find his creator. By chance he met Frankenstein's younger brother in the forest. As soon as he discovered that the boy "belongs to the enemy" he choked him. He also placed a portrait in the lap of a sleeping young girl, Justine, thereby incriminating her with his crime.

The dæmon's only request from Frankenstein was that he should create another being: a female to accompany him. If Frankenstein complies, he and his bride will stay away from other people and keep to themselves in the wild. Frankenstein saw some justice in the monster's arguments and also felt that he has a duty towards his fellow-man, so he agreed to the dæmon's request. Victor left for England to finish his work accompanied by his friend Clerval, promising to marry Elizabeth on his return. When the work on his second creation was advanced, he started to question his promise. He was afraid that they might hate each other, or that they might produce a whole race of these creatures. When the monster visits to check on the progress, Frankenstein destroyed his work. The monster swore revenge and promised to be with him on his wedding night. The following day a body was found and Frankenstein was accused of murder. He was taken to the body which he identified as Henry Clerval. He was eventually cleared of all charges and returned to Geneva in a very bad condition. Frankenstein married Elizabeth after promising her to tell her his horrifying secret the following day. Remembering the monster's threat, Frankenstein was convinced that he would be killed that night. The monster, however, kills Elizabeth instead. Frankenstein lost another family member as his father died after hearing the news about Elizabeth's death. Frankenstein had now lost every sensation except for revenge. He followed the monster everywhere which eventually led him to the Arctic region, where he was taken aboard Walton's ship.

After telling Walton his story, Victor asks him to kill the monster if he dies before he can do it himself. The ship has in the mean time been freed from the ice and pressured by his crew, Walton has decided to abandon his trip and return home. Victor's health eventually deteriorates and he dies. Just after his death, Walton finds the monster hanging over Victor's body. The dæmon speaks of his sufferings. Because of all the murders he has committed, he now hates himself. Since his creator is dead, he decides it is time that he too will rest in death. After stating that he will build a funeral pile for himself, he leaves the ship and disappears on his ice-raft in the darkness.