The attempted assassination of US President Tom Kirkman occurred in the episode Warriors during the swearing in ceremony of Congressman Peter MacLeish as Vice President. The shot came from the PLC building facing east toward the platform of the Capitol by Nestor Lozano. President Kirkman was the only one shot and targeted. Lozano's motivation was to make Peter MacLeish acquire the presidency by assassinating the current president. However, the attempt failed after Kirkman had a successful surgery at George Washington University Hospital.
In the episode The Oath, Peter MacLeish won the vote in the House 424-9 to become the next Vice President of the United States. That meant he was going to have a special swearing in ceremony at the Capitol, just like a normal inauguration but only for Vice President. Every member of Congress, the American people, and even the president came to watch the swearing in.
Hannah Wells had evidence to try and stop MacLeish from becoming VP, but she was hit by a conspirator while driving to the Capitol. An unknown man, who worked with the conspirators, tried to strangle her, but failed and took her evidence instead. Chuck Russink took care of her injuries after the accident. There he helped her find the unknown woman trying to call her by tracing phone locations and a license plate number. It was revealed that the "woman" was actually Charles Langdon living in a house in the woods but he ran away. Another unknown man showed up and tried to shoot Langdon, only to be killed by Hannah, who found a key card from him. Chuck told her the card gave access to the PLC building which faces the Capitol east toward the platform. After putting two and two together, she then realized the conspirators were going to try to assassinate Kirkman.
During the ceremony, Catalan, or Lozano enters the PLC building as normal without letting anyone in the building know of the plot. He then assembles a silenced sniper rifle which would later be used to shoot the president.
After MacLeish was just sworn in, he knew the time was right to shoot Kirkman. Unfortunately, Hannah knew where he was because of newly found evidence. She then tried to shot Lozano but it hit and ricocheted off the window, which made him flinch. After Hannah's shot, Lozano shot Kirkman on the Capitol platform, which shocked and feared the people watching in the next episode Warriors. Desmond LeGarth, another conspirator was sent out of the building as a decoy to disguise himself as Lozano. Once he was to be killed by special armed forces, Lozano would leave the building unnoticed.
Secret Service rushed Kirkman to George Washington University Hospital. They found that the bullet went straight through him and it left bullet fragments which meant he needed surgery.
Peter MacLeish became Acting President when Kirkman was in surgery. He reopened the markets, which was a bad idea after Kirkman's shooting. He also gave the shoot-to-kill order to the Navy SEALs trying to find "Lozano", (who was actually Desmond) while withstanding orders by the FBI and Chief of Staff Aaron Shore. It was later revealed that the Navy SEALs killed a decoy in the PLC building, they found out they killed Desmond, which meant that Lozano was alive. With the public thinking that he was dead, Lozano left the building after the decoy was killed. Later in the first season, Hannah kills Lozano by impaling him to reinforcing bars.
After Kirkman's surgery, he had a successful recovery. He woke up next to his wife Alex Kirkman in bed.
Meanwhile, Peter MacLeish was frustrated when Kirkman survived, after his and Lozano's motive did not result in him becoming president.
The motive was explicitly revealed by his wife Beth MacLeish who said "It's a delicate surgery Peter, if he doesn't survive it we have nothing to worry about, we are right back on track". The information of MacLeish working with Lozano and the conspirators behind the assassination and the bombing was then shown to Kirkman by Hannah in The End of the Beginning.