Episode 2.21: ? (Eko-centric)
Airdate: May 10, 2006

05/03 - Lindelof ∧ Cuse: the question-mark episode is about finding out what that question mark on that map means. That episode is a great opportunity for us to finally put Locke and Mr. Eko together, sort of a philosophical battle of faith and will that we've been hinting at all season long. [It started] back when Eko gave Locke that film that he brought across in that hollowed-out Bible, [and] this is sort of the next step in their relationship. Just as a side note, the "?" episode was originally going to be the episode that Darren Aronofsky would direct prior to Rachel Weisz's pregnancy. We thought it would be a cool shout-out to him since he made the movie Pi, which was just the symbol for pi. It ended up being directed by a guy named Deran Sarafian. It was hard to find another Darren to direct the episode, but we managed. Source: Ask Ausiello @ TV Guide
04/24 - Mr. Eko enlists Locke to help find a secret location he believes houses answers to the island's mysteries. Meanwhile, Jack and the other survivors struggle to cope with the horrific situation in the hatch. Guest starring are Adetokumboh McCormack as Yemi, Peter Lavin as Caldwell, Oliver Muirhead as monsignor, Melissa Bickerton as Joyce, Felix Williamson as Dr. Ian McVay, Nick Jameson as Richard Malkin and Brooke Mikey Anderson as Charlotte Malkin. Source: ABC
04/18 - In a flashback at the airport, Charlotte delivers a message to Eko and Libby asks if if everything is all right. Source: Aloha Oe and call sheet at SurvivorSucksForum
04/01 - Turns out that supercool black-light reveal is a major plot point that carries on through the rest of the season. "Oh yeah, big time," Damon said. "I mean, it plays huge in the finale, and even in episodes 21 through 24. Twenty-one is an episode that is very much on the sort of fundamental axis of that map, and it's a Locke and Eko story, which is going to be awesome. [Executive Producer] Carlton [Cuse] and I wrote it. That episode is just called '?' because that is the symbol Locke remembers from the map." Source: Kristin on E!Online
03/23 - Claire (and possibly another Lostaway)-centric, featuring Charlotte a 16ish Australian who suffers a near death experience. Source: SecretAgentMan at TWoP [Note: It seems that it may not be Claire-centric but that she will appear in the flashback.]