Zero Day ending explained: Who was behind the cyber attack?

A search for truth in a post-truth world ensues, as Mullen leads an investigation to track down the perpetrators and their motives.

Mullen must untangle a complex web of digital warfare and manipulation to uncover what's going on, and he's also tasked with dealing with his own declining mental state.

Wondering how it all plays out in the show, who was behind the attack and what happened to Mullen at the end of the season?

Read on for everything you need to know about the ending of the series, which comes to our screens courtesy of Eric Newman (one of the showrunners of the Narcos franchise), Noah Oppenheim and Michael Schmidt.

Zero Day ending explained: Who was behind the attack?

It turned out that the Zero Day attack was part of a wide-ranging, cross-part conspiracy, led by House speaker Richard Dreyer and also including George Mullen's own daughter, Congresswoman Alexandra Mullen.

Others involved included Congressman Jerry Benson, Congresswoman Barbara Rollins, Senator Max Trillin and Senator Pat Bishop, while those outside of the political sphere also played central roles, including billionaire Robert Lyndon and tech entrepreneur Monica Kidder.

The Cyber weapon they used was devised by the US' National Security Agency, and the plan was for a short attack in which no casualties were intended.

People around the US would be spooked by the attack, realise their collective vulnerability, and rally around the centre, bringing in a new era of bipartisanship and ending the country's extensive divisions.

Dreyer was hoping that he would be put in charge of the commission rather than George, giving him all of its power. He had promised Alexandra that this would allow him to remove the far-right elements of his party, and then he would work with her on bipartisan legislation.

Did George Mullen reveal the truth?

A number of different parties tried to convince George not to reveal the truth, including President Mitchell, who suggested the amount of corruption this would reveal could bring the whole governmental system crashing down.

Dreyer said he would step down at the end of his term and convince the other conspirators to do the same. George seemed to be going along with this, planning to just implicated Kidder and Lyndon, with the former incarcerated and the latter having fled the country.

However, in the end, while making his speech to congress, George revealed the truth, to the shock of the room and America at large. He even revealed Alexandra's involvement, and said that the investigation would need to continue to root out more of those who were involved at lower levels.

What is Proteus in Zero Day?

Throughout the series, George suffered apparent hallucinations, seeing visions of a journalist who had died after offering to help him with his memoirs, as well as ones of his son Nick, who had died during his presidency.

He also heard music and saw blurred visions, and suffered from apparent paranoid delusions. At one point, it was suggested he could have been targeted by a neurological weapon created by the National Security called Proteus which, when used, can cause these symptoms.

Was Proteus being used against George Mullen?

In the end, Valerie investigated the apparent source that could have been used to administer Proteus, but it was impossible to determine its origins.

It was suggested Proteus could have been used on George, or it could have simply been his rattled and guilty mind playing tricks on him. We don't get a definitive answer either way.

When asked about whether Proteus was being used against Mullen or not in an interview with The Wrap, co-creator Eric Newman recently said: "There will be an audience I believe that watches it and thinks he was definitely being attacked by some soundwave weapon."

He continued: "And then there will be an audience that believes he was not, and that’s very much on theme and that is intentional.

“We’re all drawing our own different conclusions from, in many cases, the same set of facts. And really we live in a world right now where having someone in a position of authority say something is one thing or the other does not really mean as much as it used to.”

What else did we learn about George's past at the end of Zero Day?

We also found out another of George's secrets during the series - not only did he have an affair with his chief of staff, Valerie, but he also fathered a child with her.

Part of his reason for standing down after his first term, it is suggested, was to stop that secret from coming out, sparing Valerie and the child from a lifetime of media scrutiny.

Why was Roger killed in Zero Day?

Mullen's right-hand man, Roger Carlson (Jesse Plemons), was killed by Lyndon's men after refusing to betray Mullen.

Lyndon was manipulating and blackmailing Roger throughout the show, but he ultimately refuses to betray Mullen when it matters most, despite the leverage Roger had against him.

Roger even gave Mullen the key to cracking the AM radio codes that solved the conspiracy behind the attack.

But this act of courage led to Roger being drowned in a bathtub by some of Lyndon’s men. The murder was staged to look like a drug overdose.

Zero Day is available to stream on Netflix now. Sign up for Netflix from £5.99 a month. Netflix is also available on Sky Glass and Virgin Media Stream.

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