Andre Kriegman (born July 17, 1982) and Calvin "Cal" Gabriel (born February 5, 1983) announce their intention to attack Iroquois High School, where they are both students,^[3] calling their plan "Zero Day". Both teens are extremely troubled, as Cal suffers from depression and suicidal ideation, while Andre is a violent and hateful individual. The majority of the film is portrayed through their video filming camera, and shows them documenting their planning and preparation for the attack, while hiding it from their friends and families.

Other scenes show the two attending Andre's birthday party, egging the house of Brad Hoff, and Cal going to prom, while Andre closes up and works at a pizza establishment. In one video entry, Cal notes the origin of the name "Zero Day": when Cal and Andre originally planned to attack the school on the first day that the temperature would drop to zero degrees Fahrenheit, after they had finished their preparations. However, this only occurred once, during which Andre was sick, and they set May 1, 2001, as the new date. Wanting their attack to have a memorable name, they agreed to keep the original title.

The two arrive at school on May 1 and prepare their plan and weapons in Andre's car. Andre says that he could never have carried out Zero Day without his partner Cal's help, a sentiment that Cal echoes. The video camera continues to film as they both run into the school through its west entrance, armed with three handguns, an M1 carbine, and a 12-gauge pump-action shotgun, all stolen from Andre's father and cousin. The pair also carried with them at least 20 explosive devices and several knives^[3] The massacre is shown through the viewpoint of several security cameras throughout the building, with audio of a 911 dispatcher playing in the background, who is attempting to persuade the two out of the attack and into surrendering peacefully. The dialogue is heard via the cellphone of student Omar, who was shot and killed.

Shooting at anyone they see and threatening and taunting several witnesses, Andre and Cal kill eleven students, and one school resource officer. They wound eighteen others.^[3] Eventually, the two see law enforcement surrounding the school after sixteen minutes of the incident in progress. After debating over whether to confront the police in gunfire, the pair decide to instead commit suicide.

Several days after the shooting, dated May 10, a group of youths film themselves driving to a memorial for those slain in the attack, including Andre and Cal. The group finds the pair's wooden crosses, before subsequently setting them on fire and swiftly escaping the area.

In order of appearance:

• Andre Keuck as Andre Kriegman
• Cal Robertson as Calvin Gabriel
• Christopher Coccio as Chris Kriegman
• Gerhard Keuck as Andre's Father
• Johanne Keuck as Andre's Mother
• Rachel Benichak as Rachel Lurie
• Pam Robertson as Cal's Mother
• Steve Robertson as Cal's Father
• Omar Walters as shooting victim Omar

The director of the film, Ben Coccio recalls that he was in a pizzeria in Brooklyn, New York on the day of the 1999 Columbine High School massacre, and saw coverage of the event on the eatery's television, commenting, "I remember thinking that I was surprised that it hadn't happened sooner." Coccio was also struck by the extent to which Eric Harris and Dylan Klebold planned that shooting, compared to the impromptu crimes of passion that typified other school shootings. Coccio became eager to address a story in a way that was very different and not exploitative, and drew upon his own views of high school as a place of tension where "anything could happen at any time". Coccio's theory of student school shooters is not that they are the most bullied, as such students tend to develop feelings of inferiority and are likely to harm themselves. Coccio feels that shooters tend to be students with feelings of superiority, and that "when other people don’t confirm that, it really gets under their skin."

None of the numerous Connecticut high schools where he sought to film the movie would allow Coccio access, and he ended up using the interior of a building at State University of New York at Purchase in Purchase, New York. The "appropriately fortress-like" exterior of the High School was New Milford High School in New Milford, Connecticut, where Coccio lived at the time.

When casting, Coccio inquired at high schools throughout Connecticut looking for teenagers interested in acting who might not have much or any professional experience. Coccio had three days of open auditions. Andre Keuck responded to an ad Coccio placed in Backstage Magazine and brought his classmate and fellow theater enthusiast Cal Robertson along to the audition. Both boys had acted in Shakespeare productions at the Stratford Avon Theater in Stratford, Connecticut. They were encouraged to improvise throughout the film's production.^[2]

An official website was made as a tie-in to promote the film, made to look like an official police report by the fictional "Essex County Sheriff's Department" on the event, describing details about the massacre that were never seen in the final film and glimpses about the perpetrators, the weapons used, and mentions that the movie actually consists of footage sent at the request of Kriegman and Gabriel's parents to an amateur filmmaker friend. The website is now archived by use of the Wayback Machine, although some features that would push the narrative that it was real have been lost.^[3]

Zero Day has received mostly positive reviews from critics, currently holding a 68% approval rating on Rotten Tomatoes based on 41 critic reviews.^[4] Despite the positive reception from critics and audiences, Zero Day was a box office bomb, gaining only $8,466 against a $20,000 budget.

• Atlanta Film Festival – Grand Jury Award 2003
• Boston Underground Film Festival – Best of Festival 2003
• Film Fest New Haven – Audience Choice Award, Best Dramatic Feature 2003
• Florida Film Festival – Grand Jury Award 2003
• Rhode Island Film Festival – Audience Award 2003
• Slamdunk Film Festival – Grand Jury Award 2003

• Elephant, another Columbine-inspired film released in 2003
• Bowling for Columbine, a documentary about gun violence in America with emphasis on the Columbine massacre
• Duck! The Carbine High Massacre, a film made in 1999 inspired by the Columbine High School massacre
• Heart of America, another film revolving around a fictionalized school massacre
• The Only Way, a 2004 independent film inspired by the Columbine High School massacre
• List of films set around May Day

• Zero Day at IMDb