The Staircase Implementation (2010)

Leonard fights with Sheldon over the setting of the thermostat, then storms out of the apartment and visits Penny, who has been listening to the argument. He tells Penny that he should have known about Sheldon from their first meeting, and his exposition to Penny of their early days is presented as a series of flashbacks that occupy most of the episode.

The first flashback begins with Sheldon's departing former roommate's dramatic warning to Leonard, and the opinion of Louie/Louise, the transvestite male then occupying the apartment that Penny moves into in the first episode - that Sheldon is "crazy". Having found the apartment, Leonard meets Sheldon who poses, at the door, one question about the periodic table and another consisting of "Kirk or Picard?" before allowing him to enter and undergo further examination, and the ensuing rigorous interview questions and intrusive conditions suggest Sheldon's literalness, rigidity, egotism, and peculiar priorities.

A flashback includes Leonard's first encounter with Howard and Raj, and the acquisition of the couch featuring what Sheldon calls "my spot".

Another flashback, introduced by Penny wondering why Leonard continues to live with Sheldon, reveals two events: Sheldon inadvertently kept Leonard from revealing a top secret rocket fuel to Joyce Kim , a North Korean spy, by angering her so much that she storms out. Later, when Leonard's botched miniature rocket and prematurely activated, Sheldon saved his life by deducing that was likely to explode while Leonard attempted to remove it from the building, and by sending it down in the otherwise empty elevator. (This incident also explains the permanently out-of-order elevator, and their invariable use of the stairs.)

First-time characters: Joyce Kim, North Korean spy; Louie/Louise, former neighbor.

Title reference: Because Leonard's rocket fuel explodes inside the elevator and destroys it, the stairway becomes indispensible.