Gardner Grayle was the Army Sergent for the infamous Platoon 13. He strongly opposed to nuclear weapons and believed they gave war a bad name. Grayle also believed that they removed the honor and valiance in battle and made it too easy to kill. In order to see how people would react to a post atomic war world, Grayle volunteered to become a test subject for a S.T.A.R. Labs project. S.T.A.R. Labs was midst preparations for a nuclear holocaust and was equipping their test subjects with atomic weapons. During this experiment, Grayle encountered the legacy of the Atomic Knights.
In the virtual world, he created a post-atomic world where he was a knight in shining armor. He was under the belief that the Atomic Knights were only a dream of the past. After emerging from the virtual world, he encountered a crisis initiated by the Injustice League. The evil league threatened the planet Rann and Earth. He emerged from S.T.A.R. Labs wearing a battle suit and declared that he was a modern knight. Deadman urged Grayle to borrow the silver-and-gold- prototype S.T.A.R. battle suit. At that point, he became the second Shining Knight and served with the modern day Seven Soldiers of Victory.
He first went into battle against Doctor Light but, the villain easily destroyed his shield. Grayle then discovered that if he joined with Metamorpho, his energy sword and freeze ray could quickly defeat the villain.
At a second encounter with the League of villains determined to bring injustice, Grayle's career as a Shining Knight quickly dissolved. During the battle, Chronos aged the battle armor so much that it turned to dust. Without his armor, Grayle resigned from his post and took a job at S.T.A.R. Labs. His colleagues said their goodbyes and claimed that he would find a disaster to prevent in the future.
Later on, Superman tapped into a virtual dream of Grayle's by accident. In the dream, Grayle was the leader of the Atomic Knights, a virtual post-World War III group. The Atomic Knights consisted of Bryndon Smith, Douglas & Marene Herald and Hollis & Wayne Hobard. When Superman realized that much of the Knights' world was illogical, Kent was ejected from the virtual world by force. Superman took to Grayle's girlfriend, Marene Herald, a S.T.A.R. labs psychologist to learn more about what was happening. Kent realized that an entire nuclear experiment was deleted from the minds of the Atomic Knights. Marene Herald had her memories restored and found the real Gardner Grayle's body still suspended in the virtual world. It was theorized that despite his physical body being dormant, his mental faculties had been expanded largely.
It was then discovered that Grayle was trying to create a post World War III world in real life so that he could emerge as a hero from that. To wake Grayle, Marene told her boyfriend that her brother was actually her husband. Grayle woke, horrified by the news. He woke right before the world's armada of nuclear missiles would have been launched due to his brain. Grayle then claimed to Superman that the entire task was misguided from the start.
Grayle's mind was expanded to such an extreme that S.T.A.R. Labs hired him on the spot, readily believing that Grayle's intellect would achieve plenty of scientific breakthroughs for the labs.
Major Story Arcs
Crisis on Infinite Earths/ One Year Later
Not unlike Cassandra in ancient Greece, Grayle had flashes of catastrophes which were upcoming in the future. Pressured by the spirit of Cassandra, he used his newly gained knowledge to recreate the Shining armor which was destroyed by Doctor Light before. Grayle then joined the Forgotten Heroes and participated in the Crisis on Infinite Earths as one of them. He was one of the Forgotten Heroes who contacted Darkseid to enlist his help against the Anti-Monitor.
After the Crisis, Grayle left the life of a superhero to work on his day job. He was a director and was paid extremely well. He had a private jet and a huge salary raise. But his visions of death and destruction did not leave his mind. Gardner looked for the famous scientist Helga Jace to find a cure.
Midst his search, he found another reason to go into action, fighting in the Outsiders to attempt to prevent the destruction of their base in Los Angeles. Major Disaster was attempting to destroy it.
Later on, Atomic Knight accompanied Geo-Force in protecting their home country of Markovia. Grayle was present when Jace betrayed them to the Manhunters. The Outsiders were then thrown into the reality of the Atomic Knights. Grayle's feelings for an Outsider named Windfall. His feelings for Windfall allowed the heroes to leave their mental prison.
In the mini-series The Battle for Bludhaven, Grayle leads an underground band of new Atomic Knights operating within Bludhaven. They worked with the organization, Roundtable towards assisting people harassed by S.H.A.D.E. and Black Baron. There are about 125 Atomic Knights including all of the Knights from the virtual world. They pretended to be refugees with the help of cloaking technology and use their armor to analyze nuclear blasts and etc. Captain Atom destroys the rest Bludhaven and the S.H.A.D.E. operatives leave.
Grayle later confessed his feelings to Windfall and they were seen together at a wedding and looked extremely happy.
Dan Turpin visits the Atomic Knights in their bunker in Bludhaven. The Knights later assist Wonder Woman into the city and go up against a Desaad possessed Mary Marvel. Mary then splits Marene's Head in half and kills her. Grayle is distraught but survives the Final Crisis.
Gardner wears an armored battlesuit that can fire heat or cold, and allows him to hold his enemies in a stasis field. His suit runs on solar energy, but can operate even in decreased light. His mind is very developed, and he has limited precognitive powers. His suit gives him super strength and durability. He can create weapons out of energy from his armor and can track others via the sensors on his suit. The suit is also adaptable to other technology. In the silver age, the character also had the power of divination, though he was cursed in that no one believed his visions of the future (which usually revolved around impending nuclear holocaust).
Grayle has visions of mass destruction which will occur in the future in the future.
Prior to the Crisis on Infinite Earths, Grayle inhabited a world torn asunder by nuclear war. This world seems to have been brought back as Earth 17.
In the story Justice, the Atom wears a suit of armor similar to Grayle's, designed to protect against Brainiac's mind controlling nanoworms.
In Countdown: Arena #2 (2007) a leader of that world's Atomic Knights appears as an alternate version of Captain Atom.