Rosen shows up at the scene of the ambulance crash, where a fellow shrink (Dr. Singh from the aforementioned "compound") explains what happened during the crash/escape we saw in the opener. Rosen immediately realizes that the escapee is a guy named Marcus Ayers, and he looks a little worried about it. Singh says Marcus has gotten even crazier during his time inside, and Rosen tells a suited FBI agent played by Mahershalalhashbaz Ali (fortunately the character's name is Nathan Cley) that they're going to need Rosen's help. Apparently Marcus lives life like he plays chess -- constantly planning and attacking. As the other guys head back to the crash scene, Rosen notices the pay phone Marcus used earlier and sees the rough outline of a chess knight scratched into the Plexiglas. Well, that's a horse of a different color.
Flashback to Marcus lecturing Rosen (clean-shaven then, too) over an outdoor chessboard in a park. Discussion of the game quickly leads to Rosen trying to disabuse Marcus of his persistent delusion that everything in the universe has intent. "Sometimes it's a flip of the coin," Rosen says, tossing a quarter on the table. Marcus insists that the universe isn't random; saying that his status as Rosen's patient is a cause and effect chain leading all the way back to his roommates pissing him off. Rosen says nobody else could possibly see it that way, and the fact that Marcus can makes him special -- like the people Rosen is calling Alphas. Marcus likes that name, taken from the first variable, acceleration. Which I thought was delta-v, but I'm just an ignorant recapper. Rosen asks if Marcus can do anything out of the ordinary (like that's not already clear), and Marcus holds up Rosen's hand to catch a dry leaf that was blowing high overhead a minute ago.
In the present day, Rosen returns to that same outdoor chessboard -- on which is sitting a single dry leaf. Marcus' mastery of seemingly unpredictable variables is such that it hasn't even blown away. But under the leaf is a coin. Rosen looks around nervously, but he's not an Alpha, so sucks to be him.