“Zero Day” Ending, Explained: Who Was Responsible for the Cyberattack — and Is Proteus Real?

The season finale of Netflix’s new political thriller Zero Day finally reveals who is behind the nationwide cyberattack that took the lives of more than 3,400 Americans.

The gripping drama stars Academy Award-winner Robert De Niro in his first-ever television series as beloved former U.S. President George Mullen. Despite stepping away from politics after the tragic death of his son, George is called out of retirement to lead the investigation into the deadly cyberattack. As the head of the Zero Day Commission, George is granted wide-sweeping and Constitution-bending power by sitting President Evelyn Mitchell (played by Angela Bassett) to find out who was behind the devastating attack — and stop them before they can act again.

During the six-episode limited series, which debuted on Netflix on Feb. 20, the Zero Day Commission struggles to nail down who is to blame. Leads initially point to the Russians, but then George zeroes in on a domestic terrorist organization known as the Reapers. After the detainment and questioning of more than 40 U.S. citizens produce no solid leads and a second, smaller cyberattack on a consumer bank leaves the nation in turmoil, George appears to be losing control of the investigation — and the country is losing its faith in him. Worse, those closest to him, including his chief of staff Valerie Whitesell (Connie Britton), his top aide Roger Carlson (Jesse Plemons) and his wife Sheila (Joan Allen), are concerned the former president may not have the mental chops to see the inquiry through.

Also impeding the former president’s progress is Speaker of the House Richard Dreyer (Matthew Modine) and George’s own daughter, Rep. Alexandra Mullen (Lizzy Caplan), who opposed his involvement with the Commission from the onset. Dreyer makes Alex the head of the Zero Day Commission’s oversight committee, attempting to undermine her father’s work, and when the Commission’s headway stalls, Dreyer urges President Mitchell to get George to resign so he can take control.

With the investigation on the brink of crumbling around him, George gets a break in the case — thanks to his troubled aide Roger, who is killed by men tied to a corrupt billionaire for not turning on his boss. Before his murder (which is staged as a drug overdose), Roger leaves George a note detailing secret communications he discovered occurring on long-range radios. The posthumous tip leads to the Zero Day Commission cracking the comms network’s secret code and staging a sting operation, landing them their first major arrest in the case: Monica Kidder (Gaby Hoffman), the chief executive of global tech giant Panoply. But before Kidder can be questioned on her motives and accomplices, she’s found dead in her cell from an apparent suicide.

As the final episode begins, the Zero Day Commission’s work appears to be complete. Kidder is believed to have launched the cyberattack as retaliation for the Federal Trade Commission’s investigation into her company for antitrust activity, and killed herself to avoid answering any questions about it. George, however, isn’t completely convinced. As he continues to pull at loose threads, he discovers that the individuals behind the Zero Day attack are not only within the country’s own government — but within his own home. He is left grappling with the choice to reveal the truth, or protect his own family.

Read ahead for the Zero Day ending, explained — down to every last nail-biting moment.

Warning: Zero Day spoilers ahead!

How does Zero Day end?

The sixth and final episode of Zero Day begins with the apparent mastermind of the Zero Day attack, Kidder, dead in her prison cell of an apparent suicide. Speaker Dreyer is in his office, telling his media team to plant a quote in the news that places the blame for the attack squarely on Kidder, when Alex bursts in demanding answers. Their tense exchange reveals that while Kidder was responsible for deploying the malware that carried out the Zero Day attack, it was actually Dreyer, Alex and a group of congressmen and women who were the orchestrators of the plan. While Alex wants them to own up about their involvement, Dreyer insists that Kidder — and Kidder alone — needs to take the fall.

Alex leaves and phones her mother, who is at the family’s upstate New York home with George. She shares that she is on her way from D.C. to see them, and needs to talk to them both. Alex then issues an ominous warning to her parents (“Be careful”) as the second Zero Day attack hits — wiping out the power grid, cellular networks, FAA and mass transit. Unlike the first attack, though, this one is not just one minute long — it’s persistent.

During the blackout, Secret Service officers come to take George and Sheila to a safe house. But as they attempt to leave the Mullen family home, violent protestors attack their vehicle, pulling the protective officers into the mob and launching Molotov cocktails towards the former president and his wife. George and Sheila are saved when CIA director Jeremy Lasch (Bill Camp) appears out of the woods, dispersing the crowd with gunfire shot into the air. In the car, Lasch sends out a fake message on the long-range radio stating that George has been killed, before revealing to George that he and President Mitchell have had reason to believe that the Zero Day attack came from inside the government. Now, they need George and the Zero Day Commission to confirm who.

As the country is still plunged in darkness and silence, George goes to work — first confronting his daughter Alex before arranging a private meeting with Dreyer. Alex confesses to her father, confirming that it was Dreyer’s plan and that their intent, as she understood, was just to scare people, not harm them. The goal, according to Alex, was to remind the country of how fragile and vulnerable it was, silence the extremists and conspiracy theorists in the aftermath and be able to move forward with positive change.

George — who is presumed dead by the public — then goes to meet with Dreyer, who attempts to blackmail him into keeping his involvement a secret. Dreyer says that in return for not exposing him, he will step down when his term is up, as will the others who were involved in planning Zero Day, and George can place all the blame on Kidder and Robert Lyndon, the corrupt billionaire who funded the attack. George refuses to negotiate, until Dreyer reminds him that treason is a capital offense — and his only living child, Alex, is implicated.

In the days leading up to his presentation of the Zero Day Commission’s findings before a joint session of Congress, George is left with an impossible dilemma: Expose the true perpetrators of the Zero Day attack and turn in his own daughter, or keep quiet about the government’s involvement and protect his family — and the country, who President Mitchell believes won’t survive a government conspiracy at this level.

Does George reveal who was truly behind the Zero Day attack?

Prior to his speech, George appears prepared to stay silent. Valerie delivers a copy of the Zero Day Commission’s findings to Dreyer, who is pleased with the conclusions. And when questioned by Carl Otieno (McKinley Belcher III), the Zero Day Commission’s lead investigator, about additional angles that may have been uncovered, George shuts him down.

“This is what we’re going with,” George tells him definitively.

But as George is delivering his findings to Congress and the nation, he is interrupted by an auditory hallucination that reminds him of his son Nick’s death. After the momentary pause, George pivots away from his prepared speech and reveals that, just like you can’t escape the memory of someone you love who you’ve lost, you cannot escape the truth.

“The Zero Day attack was a conspiracy led by House Speaker Richard Dreyer,” George says to the shocked audience — before naming all of the congressmen and women who participated, as well.

What does this revelation mean for George’s daughter, Rep. Alex Mullen and Speaker Richard Dreyer?

Even before George exposed the truth about the Zero Day attack, Alex was prepared to turn herself in and accept the consequences. Though she did not attend her father’s speech, she left him a letter stating her intentions to come clean and writing how she planned to “confront hard truths, not hide from them.” George reads her letter out loud during his address, before pointing the finger at her co-conspirators.

As George concludes his congressional address, he implores President Mitchell to use every Constitutional power to bring justice upon the Zero Day traitors — including its ringleader, Dreyer. Dreyer, who was seated behind George as he exposed him, is furious and accuses George of destroying the country.

“Every time we can do the right thing, it’s another chance to save it,” George responds.

Related: Zero Day Cast Reveals How AOC, the Clintons and Other Political Figures Influenced Their Characters (Exclusive)

What about Proteus?

While investigating the Zero Day attack against the country, George believed he was being targeted by a neurological weapon called Proteus. The weapon was originally developed by the CIA and was able to inflict a traumatic brain injury on its targets from a distance, but the program had been shut down by George himself when he was in office. However, after some digging by Valerie, George learned that the symptoms he had been dealing with — such as visual and auditory hallucinations and memory loss — could be the result of a Proteus attack.

With George’s mental acuity in question, Sheila is subpoenaed to testify in front of the Zero Day Commission oversight committee. Dreyer asks Sheila directly about George’s state of mind, which she describes as “sound,” silencing any outside doubts about his decision making for the time being.

However, George’s concerns about being targeted by Proteus only grow when he discovers what appears to be a small device inside a bird feeder on the edge of his property. After being thoroughly investigated by the lab, though, it is deemed to be simply “debris of indeterminate origin.” Valerie chalks up George’s symptoms to the pressure and stress of leading the commission, not a result of Proteus — but they don’t have a definitive answer either way.

“Top-secret neurological weapon or just a tired old man with too many demons?” George wonders. “Does it really matter?”

Where does George go from here?

With his work leading the Zero Day Commission complete and the true perpetrators of the attack exposed, George hands off the work of bringing them to justice to President Mitchell. Following the bombshell revelations, he returns to his home in upstate New York, which is dark and empty. George goes to his office, where he burns a copy of his untitled memoir — abandoning the project he had been working on prior to the Zero Day attack. Though the political thriller began as the country was plunged into chaos and catastrophe, it ends with a moment of calm, as George pauses his morning run with his dog to overlook the Hudson River and reflect.

Will there be a Zero Day season 2?

Zero Day was ordered by Netflix as a six-episode limited series in March 2023, Deadline reported. There has been no indication that a second season will be forthcoming.

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