Paul Adelstein (Paul Kellerman) returns briefly in a key political role, adding nostalgic weight.

Knepper chews scenery like vintage ham, but he also shows profound vulnerability. The revelation that Michael framed T-Bag to use him as a pawn, and T-Bag’s subsequent vengeance on Poseidon, turns the character into a tragic anti-hero. He is creepy, charismatic, and quotable. For pure entertainment value, Knepper is second only to Miller.

The biggest gamble of Season 5 was replacing the iconic villainy of William Fichtner (Mahone) or Robert Wisdom (Lechero) with Mark Feuerstein’s "Poseidon." Known for sitcoms ( Royal Pains ), Feuerstein seemed an odd choice. But that was the point. Poseidon is a monster hiding in a sweater vest. He is a rogue CIA agent who kidnapped Michael, forced him to run black ops, and then erased his identity.