Immersive expansion delivers more amazing content.
Upon installing the expansion, you can load up a saved game or create a new character. After 24 hours of in game time, you receive a notice that a mysterious portal has opened up near Bravil. This portal leads to the Shivering Isles, a strange place ruled by Sheogorath, the Daedric price of madness.
The main quest line starts off in a town called Passgate. Access to the Shivering Isles is blocked by two games guarded by a gigantic abomination called the Gatekeeper (so original). Once you figure out how to get passed the Gatekeeper, you are quickly summoned to have an audience with Sheogorath himself. After which, the you become sort of the right hand man for him, trotting about the Islands accomplishing various tasks.
In true Oblivion fashion, there are many side quests to complete which can be found around the various settlements. The new island is also chock full of dungeons to explore and loot. You may also notice various characters from Tamriel have moved to the Shivering Isles to live, some of them even recognize you. There aren't really any new factions to join other then the faction associated with the main quest line.
One of the new things in Shivering Isles is a sort of crafting system. Throughout the land you will acquire madness ore and amber which can be taken to smiths in New Sheo to be fashioned into weapons and armor. If you have found one of the many matrixes scattered about, they can fashion magical versions of the weapons and armor for you.
Overall, if you are a fan of Oblivion, I highly recommend this expansion.