Warning: SPOILERS ahead forZero Day, as well as references to drug addiction and suicide.Robert De Niro’s first major TV show,Zero Dayputs the actor in the role of former US president George Mullen, who’s tasked with investigating a deadly cyber attack on the country. Throughout the series, Mullen is dogged by images and sounds that seem to indicate some past trauma he’s still haunted by. Indeed, he is. Mullen is wracked by unresolved grief and guilt over the death of his teenage son, Nick. It isn’t immediately clear inZero Dayhow Nick Mullen died. We only know that it took place before the events of the Netflix show.

However, as the stories ofZero Day’scharacterscontinue,Mullen experiences flashbacks that reveal the details of what happened to his son. It becomes increasingly clear that many of his actions inZero Dayare partly motivated by the feelings he is still processing in relation to Nick. George Mullen’s personal and patriotic motives become entangled, as they often do in thebest political thrillers. As the commission headed by De Niro’s character comes under increasing pressure from all directions leading up toZero Day’sending, it becomes clear that problems from Mullen’s family life are interfering with an investigation on which the safety of his country depends.

Robert De Niro and Angela Bassett in Zero Day

George Mullen’s Son, Nick, Died When He Was Still In Office

Mullen Then Gave Up His Chance Of Reelection And Stood Down As President

In one ofZero Day’s first scenes, we learn that George Mullen’s son,Nick, died during his father’s first term in office as President of the United States, several years before the events of the series take place. When an editor from the publishing house looking to release Mullen’s memoirs asks him why he didn’t stand for reelection, he replies sharply, “Maybe we were just too busy burying our son.” It’s clear that Nick’s death happened towards the end of Mullen’s four years as president, as it’s made to sound as though his campaign for reelection was already being discussed.

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What’s more, in a flashback sequence during the show’s fourth episode, Mullen is seen discovering thathis son has died within the White House itself. He ascends a carpeted spiral staircase that’s clearly not in his private residence, and walks down a corridor decorated with historical portraits, making it easy for us to identify the place of Nick’s death as the US president’s official residence. The Mullen family was still living there when Nick died, because Mullen was president at the time.

Robert De Niro, Jesse Plemons, Connie Britton, and others in Zero Day

How Nick Mullen Died Prior To The Events Of Zero Day

He Was Killed By A Fatal Overdose In His Bedroom

It’s George Mullen having to identify the dead body of his secretary Roger that triggers his flashback to the scene of his son’s death. He was asked to do the same thing by Secret Service personnel who discovered Nick’s body in his bedroom at the White House. We see Mullen entering the bedroom and finding his dead son, apparently the victim ofa drug overdosewith a needle still in his arm. In another flashback, Nick is shown cooking heroin when his father walks in on him and physically stops him.

“He OD’d. You may think he killed himself but there’s no proof of that.” - George Mullen on his son’s death inZero Day

Robert De Niro interrogating Ben Stiller in Meet the Parents

Nick seems to have had a heroin addiction that eventually killed him, although his sister Alex suggests that his fatal overdose was, in fact, a deliberate suicide. George disputes his daughter’s claim, telling her, “You don’t know that. None of us do. He OD’d. You may think he killed himself but there’s no proof of that.” Nick’s death being a suicide rather than an accident isn’t ruled out. It’s just that there was nothing to indicate that it was deliberately self-inflicted. Either way, the physical cause ofhis death certainly appears to have been a heroin overdose.

How Nick’s Death Affects George Mullen In Zero Day

He’s Plagued By The Trauma Of What Happened Along With Feelings Of Guilt

George Mullen has obviously been deeply affected by Nick’s death, even if he isn’t forthcoming about his feelings on the tragedy. The most immediate effect it had was preventing him from running for a second term as president. While various characters in theZero Daysuggest that there were other reasons for him stepping down after one time, Mullen himself is unequivocal that Nick’s death was the reason.

He’s clearly haunted by the trauma of the event, to the extent that when his brain is damaged by the perpetrators of the Zero Day attack, he continually hears thesong “Who Killed Bambi?”by the Sex Pistols playing in his head. This was the song playing on Nick’s stereo in his bedroom when his father saw his dead body for the first time.

Zero Day official poster

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In addition,Mullen apparently feels guilt about his son’s death as well as trauma. It’s a vague sense of “regret” that motivates him to return to public office as head of the Zero Day Commission, a sentiment undeniably associated with Nick. He often treats his secretary Roger like a son during the series, and admits after Roger dies that he wanted to “do better” with him than he had done with Nick.

It’s also a vision that Mullen has of Nick that convinces Mullen to tell the truth about the cyber attack inZero Day’s series finale, even if it means bringing down his own daughter in the process. “Every time we can do the right thing it’s another chance to save it,” he tells the Speaker of the House. Although he’s ostensibly talking about his country, he could just as easily be referencing the guilt he feels about his son’s death.