Right, if I remember correctly, microfilm is mentioned when Robb discusses the recording of the conversation with Pash, but not during the conversation itself in the film. The book discusses this cock and bull story, as well as the innumerable other versions of the Chevalier incident, in very great detail throughout the book as the incident, the different versions of it, and its consequences weave in and out of Oppie's life.
The cock and bull story - what he told Pash - was, essentially, that somebody at the Soviet Consulate with experience in microfilm had approached intermediates, e.g., Eltenton, that he could transmit information to the Soviets. Eltenton had told somebody who knew someone on the Manhattan Project, and that person approached three scientists on the project (two at Los Alamos, one at Berkeley, supposedly). Oppenheimer refused to name any of these people because he had judged they were all innocent, and that only Eltenton was worth looking at. Now the incident according to Oppenheimer's post-war testimony was that Eltenton had approached Chevalier, Chevalier approached Oppenheimer, described in very vague terms that he knew somebody who could transmit information, Oppenheimer cut him off saying "that would be treason," and that was the end of that. Nothing about someone at the Soviet Consulate, nothing about microfilm, and nothing about Chevalier or anybody else approaching three scientists on the project besides Oppie. All of that was a total fabrication by Oppenheimer to Pash to avoid implicating Chevalier... or was it? What gets me personally is that there's a lot more truth to Oppenheimer's "fabrication" than appears at first glance. There was in fact someone on the Soviet Consulate, Peter Ivanov (who in all likelihood had microfilm experience, but it's not specifically mentioned), who approached Eltenton asking if he knew anyone at Berkeley who'd be willing to give up information. And Ivanov asked Eltenton about three specific people (Oppenheimer, Lawrence, and maybe Alvarez), upon which Eltenton said he only knew Oppenheimer, and only through Chevalier. It's possible that Eltenton told Chevalier about three scientists and Ivanov, and Chevalier relayed that info at length to Oppenheimer, and the story became garbled in Oppie's mind by the time he told Pash about it many months later. There's a lot more complexity to the Chevalier incident in all its different forms (all discussed in the book in detail), but that's the gist of it that's relevant here.