Martin informs you of a very bold plan to obtain the final item required to open the gate the Mankar Cameron's Paradise - a Great Sigil Stone obtained from a Great Oblivion Gate.
You will need to inform the Countess Carvain of this daring plan, and prepare for battle at the Temple of Talos. The Countess accepts the plan, and she will order her guards to prepare for battle.
You will need to defend Martin until the Great Gate opens. If you did the Allies for Bruma quest, you should have additional meatshields to keep Martin safe from 3 increasingly larger waves of Daedra from the lesser gates and the Great Gate itself.
Be sure to stock up with a lot of Healing potions (10-15) before entering this battle. You won't have a second chance later, and the follow-up quest in the Great Gate itself doesn't allow for much margin of error.