Alright, I'll answer that. Put on ya' readin' glasses.
Emperor Uriel Septim VII, the current reigning Emperor, arrives at his palace prison escorted by several Blade bodyguards to escape through a secret exit through the Imperial Sewers. By chance, the exit is located in the cell occupied by the player character, who is in prison for reasons that are never revealed. Through conversation with the main character, the emperor relates that assassins, later revealed to be a part of a Daedric cult known as the Mythic Dawn, have killed Uriel's three sons and are now after him. He is then led off into the catacombs beneath the palace. The protagonist follows, and, after being cut off from the Emperor and his guards, the main tutorial takes place, which teaches the basic game mechanics and collects information that eventually leads to the game suggesting which class the player should choose near the end of the dungeon. The player is asked at the end of the dungeon whether the class they chose was correct. In keeping with the open nature of the game, the player is not required to follow this suggestion, and may pick any pre-made or custom class.
At the end of the catacombs, the protagonist meets up with the guards and Septim again, and they are quickly overwhelmed by assassins, which results in the player taking on the task of guarding the Emperor while the surviving Blades engage the enemy. While awaiting the result, Uriel entrusts the protagonist with the Amulet of Kings, a special amulet that can only be worn by those of the Septim bloodline, and orders him/her to take it to a man named Jauffre. Immediately afterwards, an assassin ambushes and kills the emperor before being defeated. The surviving guard, Baurus, questions the protagonist, and explains that Jauffre is the Grandmaster of the Blades, and can be found at Weynon Priory, near the city of Chorrol. The protagonist then leaves the sewers and begins his journey in the land of Tamriel; it is the player's choice to either follow these orders or go his or her own way.
If the player chooses to continue the main storyline, and ventures to Weynon Priory, it is revealed that the Emperor's death has allowed multiple gates to Oblivion to open, and a Daedric invasion is to begin as a result. The only way to close down the gates permanently is to find someone of the Septim bloodline to retake the throne and re-light the Dragonfires in the Imperial City. Fortunately, it is also revealed that there is indeed still an heir to the Septim throne: an illegitimate son named Martin Septim, who resides in Kvatch. The protagonist then must venture to the city and bring him to Weynon Priory. Ultimately, in the end, Tamriel is almost destroyed by Mehrunes Dagon, and his army of Daedra. (The site stopped here because of spoilers)
Other than the main storyline, the player has many other choices to choose from. Such as, joining guilds, doing side quests, stealing, etc. Even if you've played Oblivion 200+ hours the player can still find something to do. Like collecting all of the weapons they can (500+), raising money to buy homes, or killing everyone that they can! Also within the game are hundreds of spells. Ranging from a simple unlocking spell, to a gigantic fireball, and everything in between. The player can even join the "Mage's Guild" to unlock more powerful magic. If that isn't what they would like to do, they could join the "Fighter's Guild" the shady "Thieves Guild", the mysterious "Dark Brotherhood" All in all, Oblivion will last you a LONG time.
I love to copy and paste.
"To those who claim they will die for their country...
What the %#($ is wrong with you!?
Make the enemy die for theirs!"