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The Descent is the twentieth episode of the fourth season of Alias, and the eighty-sixth episode overall.
Starring: Jennifer Garner (Sydney Bristow), Ron Rifkin (Arvin Sloane), Michael Vartan (Michael Vaughn), Carl Lumbly (Marcus Dixon), Kevin Weisman (Marshall Flinkman), Mia Maestro (Nadia Santos), Greg Grunberg (Eric Weiss) (credit only)
- Sophia Vargas is discovered to be Elena Derevko and that she led the Covenant and was responsible for recent events such as the Sloane doppelganger and the ruse of Vaughn's father.
- The DSR is ransacked of its Rambaldi artefacts. Assembling them based on a manuscript to bring forth Rambaldi's final prophecy was Sloane's objective when he and left with Nadia to find the Sphere of Life. He warns that in Elena's hand, it could be apocalyptic.
- Dixon reveals that while at Elena's base, he saw someone shackled being put in the back of a truck. It was Irina.
- Vaughn plans to propose to Sydney.
- Sloane forms an alliance with Elena.
- Dixon is shot and Director Chase watches over him by his bedside.