You're no longer forced to play the main quest. You start off in Anvil (having just arrived by ship from another province). The Emperor is still alive and Kvatch has not yet been invaded. To start the main quest, simply commit a crime and surrender to an Imperial Watch Captain or Commander. The main quest will not start if you are arrested by a regular guard.

This also adds several other things, so read below...

Things worth mentioning about this plug-in:
a. Fully lip-synched and spoken dialogue, as usual.
b. You choose your race, birthsign and class straight from the beginning of the game, you cannot change them after that though.
c. You can choose a different starting level. This will affect both leveled content (leveled monsters, leveled loot etc.) and your own attributes and skills.
d. You can choose to become a vampire straight from the start. The game starts at 1:00 A.M. giving you plenty of time to feed or find shelter from the Sun. You start with 100% vampirism.
e. Added a new ship in Anvil (the one you arrive on) with its own name and captain.
f. Upon your arrival in Anvil, you will be wearing peasant clothing instead of prison clothing. You will also have 17 gold, a rusty dagger and an apple in your inventory. (Don't say I didn't give you anything.)
g. The Emperor is still alive and you can find him in the Imperial Palace guarded by his loyal Blades.
h. The Dragonfires are still burning in the Temple of the One. They will remain lit until the Emperor is dead.
i. Kvatch has not yet been invaded.
j. All references to the Emperor's death (dialogue lines, rumors, books, newspapers, letters etc.) have been disabled. Once again, they will be re-enabled once the Emperor is dead.
k. You can go to the Imperial Palace and kill the Emperor yourself, but then you will no longer be able to play the main quest. You'll also earn a 100000 gold bounty.
l. To start the main quest, you must break the law and surrender or get caught by an Imperial Watch Captain, Centurion or Commander. (examples: Adamus, Lex, Itius, Audens etc. from the Imperial City) Please note that some of them will leave the Imperial Watch at some point in the game and but will probably be replaced by others. Upon your arrest, you'll get prison clothing and wrist irons, as in the beginning of the game. All other possessions, with the possible exception of items marked as "quest items", will be sent to the Evidence chest.
m. If you have reached the point where you have accepted the Dark Brotherhood's Valen Dreth assassination contract, you will no longer be allowed to start the main quest, even if you surrender to a high-ranking member of the Imperial Watch.
n. Baurus, Glenroy, Captain Renault and the Emperor are no longer scripted to be immune to health damage.
o. If you kill Baurus, Glenroy, Captain Renault or the Emperor before they're supposed die according to the original quest, you'll be given the same message from Morrowind: "With this character's death, the thread of prophecy is severed etc."
p. You must start the main quest before you can play any part of the "Tools of Kagrenac" quest. This is due to an obvious sign of the Emperor's death in the quest.
q. You can now keep Glenroy and Renault's Akaviri swords as long as you drop them before talking to Baurus.
r. You can steal the Emperor's clothes when Baurus isn't looking. (i.e. detecting you)
s. Several hidden dialogues, especially in the beginning of the main quest have been unlocked.
t. Fixed a minor dialogue bug from Chorrol.
u. You can now keep Baurus' key and the Sancre Tor key if you want. Instead of automatically being removed from your inventory, they will simply lose their "quest item" tag when you no longer need them.
v. Hitting the Emperor or his bodyguards during Charactergen will no longer raise your bounty since there are no official Imperial guards around anyway.
w. Warning: You may have to initiate the Charactergen quest before you can play some of the official Bethesda Softworks plug-ins.