Wikipedia summarizes the LOST seasons as follows:
The first season begins with the aftermath of a plane crash which leaves the surviving passengers of Oceanic Airlines Flight 815 on what seems to be a deserted tropical island. Their survival is threatened by a number of mysterious entities, including polar bears, an unseen creature that roams the jungle (the "Smoke Monster"), and the island's malevolent inhabitants known as "The Others". They encounter a French woman named Danielle Rousseau who was shipwrecked on the island 16 years before the main story and is desperate for news of a daughter named Alex. They also find a mysterious metal hatch buried in the ground. While two survivors try to force the hatch open, four others attempt to leave on a raft that they have built. Meanwhile, flashbacks centered on individual survivors detail their lives prior to the plane crash.
The second season follows the growing conflict between the survivors and the Others, and continues the theme of the clash between faith and science, while resolving old mysteries and posing new ones. A power struggle between Jack and John over control of the guns and medicine located in the hatch develops, resolved in "The Long Con" by Sawyer when he gains control of them. New characters are introduced, including the tail-section survivors (the "Tailies") and other island inhabitants. The hatch is revealed to be a research station built by the Dharma Initiative, a scientific research project that involved conducting experiments on the island decades earlier. A man named Desmond Hume had been living in the hatch for three years, pushing a button every 108 minutes to prevent a catastrophic event from occurring. One of the crash survivors betrays the others and the cause of the plane crash is revealed, as the Others capture Jack and several castaways and take them hostage.
In the third season, the crash survivors learn more about the Others and their long history on the mysterious island and the fate of the Dharma Initiative. The leader of the Others, Benjamin Linus, is introduced as well as defections from both sides pave the wave for conflict between the two. Time travel elements also begin to appear in the series, as Desmond is revealed to be "unstuck" in time and seeks this ability to alert his longtime girlfriend Penny to rescue the survivors. The survivors make contact with a rescue team aboard the freighter Kahana. At the end of the season however, the show undergoes a format switch, with flashbacks being abandoned for "flash forwards."
Season four focuses on the survivors dealing with the arrival of people from the freighter, who have been sent to the island to reclaim it from Benjamin. The survivors form an alliance with several members of the freighter, who are revealed to have ties to the island and who realize the secrets of the island must remain hidden from those who seek to abuse them. They eventually convince Benjamin to help them, in "moving" the island backwards through time via a time travel mechanism hidden within the bowels of the island, while six castaways escape back to civilization. While seasons one through three focused on flashbacks alongside the main story, season four continues with the "flash forwards" established in the season three finale and slowly reveal the identity of those who escaped the Island (dubbed "The Oceanic Six" by the media) and their encounters with Benjamin Linus, who informs them of a death of a seventh character, which necessates them returning to the island.
The fifth season follows two timelines. The first takes place on the island where the survivors who were left behind erratically jump forward and backward through time until they are finally stranded with the Dharma Initiative in 1974, while Benajmin Linus and later, John Locke, to the present. The second continues the original timeline, which takes place on the mainland after the Oceanic Six escape, and follows their return to the island on Ajira Airways flight 316 in 2007 (three years after they escaped). Some passengers on the Ajira flight land in 1977 and some remain in 2007,. The ones who in 2007 and the surviving Others unite to a dangerous mission to assassinate Jacob, the island's protector. Those who land in 1977 reunite with the other survivors who have lived for three years with the Dharma Initiative, only to fail to attempt to change past events in order to prevent the Oceanic plane from crashing in the future.
In the sixth and final season, the main storyline follows the survivors, reunited in the present day, as the death of Jacob allows for his brother, the Man in Black, the human alter ego of the Smoke Monster, to take over the island. Having assumed the form of John Locke, the Smoke Monster seeks to escape the island and forces a final war between the forces of good and evil. The final season also features a "flash-sideways" narrative, that follows the lives of the main characters in a setting where Oceanic 815 never crashed though additional changes are revealed as other characters are shown living completely different lives than they did. In the final episodes, a flashback to the distant past shows the origins of the island's power and of the conflict between Jacob and the Man in Black, who are revealed to be twin brothers with Jacob desperate to keep his brother from leaving the island after he is transmogrified by the power of the island and becomes the smoke monster. The Man In Black is killed as one of the survivors becomes the new caretaker of the island, several of the survivors die in the conflict or stay on the island, and the remaining escape the island once and for all. The series finale reveals that the flash-sideways timeline is actually a form of limbo in the afterlife, where some of the survivors and other characters from the island are reunited after having died because their time on the island had been the most important part of their existence. In the end, the survivors are all reunited in a church where they "move on" together.
