Warning: SPOILERS for Red Sparrow are in play. If you haven't seen the film yet, bookmark this article and come back once you're current.
As any good espionage thriller tends to do, Red Sparrow engages in quite a few twists and turns throughout its narrative. In particular, the film's third act totally hinges on revelations and actions spurred on by the knowledge they contain, leading to a good portion of the film's finale winding through a spy's labyrinth of betrayal and success. Which is why it seems like a good idea for us to run through the major building blocks of Red Sparrow's shocking finale in order to help show the big picture of what happened, what it means, and why those moments were so surprising to start. So we'll give one last warning before going forward: once you've cleared the next paragraph, you are deep in spoiler country -- so jump out while you can.
Red Sparrow's Central Mystery
CIA agent Nate Nash has an informant in the Russian government, whom we see him botch a meeting with in the beginning of Red Sparrow. After this compromising position, the Russians are curious as to the identity of their mole. It is then decided by Ivan Egorova to send his niece, and recent Sparrow School graduate, Dominika in to seduce and extract that information from Nate. Throughout the film, the key mystery at the heart is "Who is MARBLE?" -- the codenamed operative working with Nate to pass state secrets. Sure enough, in the third act, Dominika obtains this information from MARBLE himself, as he reveals his identity during her recovery from an attack sanctioned by her uncle and his superiors.
The True Identity of MARBLE
The man who reveals himself to be MARBLE is Jeremy Irons' General Vladimir Korchnoi, one of the figures who we previously though was working against Dominika's efforts throughout Red Sparrow. Korchnoi explains to Dominika that his efforts with the U.S. government are spurred on by a personal vendetta, as his terminally ill wife wasn't allowed treatment by an American doctor, leading to her ultimate demise. With his progressively crumbling allegiance fully shattered, Korchnoi aligned himself with the Americans, and ultimately suggests that Dominika reveal his secret to her uncle so that she can take his place as the new source of high ranking intelligence for the CIA. Soon after that, Dominika makes her play to give MARBLE up to her Russian superiors, although she puts a unique twist on her findings.
As Red Sparrow's central mystery draws to a close, Dominika reports to the Russian Ambassador in Budapest that she knows MARBLE's identity. However, she will only report this knowledge to Colonel Zyuganov, the third member of the Russian trio handling Dominika. This knowledge then leads to a handoff between the CIA and Russian intelligence, to be set at an airfield in Budapest. It is there where it's revealed that Dominika framed her uncle Ivan as MARBLE, rather than give up Korchnoi as the real mole.
We then see a flashback showing Dominika's previous actions of collecting a whiskey glass from Nate's apartment and opening a bank account in Vienna are now presented as they truly were: actions that helped plant evidence of her uncle's would-be treachery as revenge for forcing her into Sparrow School in the first place. As they meet during the handoff, her uncle claims she's just about killed him, to which Dominika smiles and replies, "Didn't I do well, Uncle?" Ivan proceeds to make his way to the CIA's custody and is shot dead by a sniper stationed in the grass, while Dominika is rapidly whisked away to the helicopter waiting for her.
After her retrieval from Budapest, Dominika returns home to Moscow, and we see her spending time with her sick mother. Of course, this safety comes from the fact that Dominika has now received a promotion and quite a bit of fanfare from the SVR for her efforts in outing her uncle as the supposed mole. We see her celebrated at a sort of press conference, with Korchnoi and the matron of the Sparrow School looking on with pride. Still, the final moments of Red Sparrow do indicate that the relationship between Dominika and Nate isn't completely dead, as she receives a phone call playing a familiar tune: the music she and Nate listened to on their first meeting. She looks out the window expectantly and smiles, leading us all to think that this story is only getting started. With two more books in the series, we couldn't be more right.