Communications have been lost with the Battlespire, the Imperial Battle College's elite training center in a pocket realm in the vast chaotic emptiness of Oblivion. An agent, an acquaintance, is sent to investigate. She doesn't report back. So they send you.

You enter to find yourself trapped, your return through the Weir Gate blocked by a magical Daedric Sigil. The staff of the Battlespire are all dead, and the facility is crawling with Daedra. In addition, four of the arcane anchors holding the Battlespire in place have been disconnected. You must restore the anchors, or the Battlespire will be destroyed.

You can't go back to Tamriel. Once you have restored a teleport device, you can only search deeper into the occupied Battlespire laboratories and administrative center.

Fortunately, the agent who preceded you has left you reports of all she has learned. She has gone on ahead to scout and track down the traitor who betrayed the Battlespire.

From your agent's notes, you discover that a human servant of the Daedra Lord Mehrunes Dagon has induced a Battlespire mage to turn traitor and open a gateway for an invasion of Daedra. Your only hope is to fight your way past the Daedric troops, guards, and captains, steal arcane keys from the bodies of your enemies, and pass through the Beachhead Portal the Daedra used to invade the Battlespire.

This will lead you directly into the enemy's camps and routes of march -- into the myriad pocket worlds that float in the voids of Oblivion. At best, you might find a way back. At worst, you can avenge yourself on the Daedra invaders and the traitor who opened the way into the Battlespire.

Through the Portal you find yourself in the Soul Cairn -- a Daedric realm occupied by lost souls. You must fight your way past the maddened revenants and the Daedra garrison troops to make your way to the next pocket world -- the Shade Perilous.

In the Shade Perilous, you discover that a number of Daedric realms have been invaded and occupied to provide travel corridors from Mehrunes Dagon's invading army base to the Battlespire. To pass through to the next Daedric realm along the invasion path, you need the help of two Daedra lovers, Jaciel Morgen and Deyanira Katrece, victims of the invading Daedric army. Only Deyanira Katrece's tragic sacrifice can shake Jaciel Morgen from her despair enough to help you teleport to your next objective -- the Chimera of Desolation. Jaciel Morgen also promises to come at your call when you reach Mehrunes Dagon; she pledges to help you take your revenge on him.

The Chimera of Desolation appears to be a great island floating in the void. Here you find yourself the target of a Wild Hunt. Only with the help of the old mad hermit of the island can you find the Armor of the Savior's Hide and the Spear of Bitter Mercy to turn the tables on the Daedra huntmaster and his Daedric dogs chasing you. On the dead huntmaster you find the key to the gate that leads you to the next realm.

The Havoc Wellhead is the staging area for the Daedric invasion of Battlespire. Here the ambitious and competing rival captains of Mehrunes Dagon's armies are mustered with their respective troops, preparing to use the Battlespire as a bridgehead to invade Tamriel. You must play the rivals off against one another to get the keys to the next realm. You also get the opportunity to kill the mortal sorcerer who treacherously helped Mehrunes Dagon stage his invasion of the Battlespire.

The next and final realm is full of giant stone islands floating above a sea of lava. The floating stone islands are connected by flimsy bridges and guarded by mighty Daedra warriors. In the center of the realm is the Hunting Lodge of Mehrunes Dagon. You must enter the Hunting Lodge and confront Mehrunes Dagon, using the lore you have wheedled from Daedric informants, wearing the Armor of the Savior's Hide, and striking Mehrunes Dagon with the Broadsword of the Moon River (taken from the body of one of the Daedra Lord's most fierce and faithful bodyguards).

You call on Jaciel Morgen. She comes and distracts Mehrunes Dagon long enough for you to strike him, hard and true. The Daedra Lord is cast far out into the trackless void of Oblivion, and will be a long time finding his way back to his realm and his minions. You rescue your fellow agent held captive here, then return to Tamriel to receive the grateful praise of your emperor.