Navy SEAL James Reece kills investment banker Marcus Boykin in Wyoming, marking the first strike in a hunt for payback. Three months before, Reece's SEAL troop falls in an ambush in Afghanistan. The mission, based on tips about a target, results in the deaths of his whole team, except for Reece and Boozer. Back at base, Reece faces questions by NCIS agents who seem to doubt his choices. While in the hospital for a hit to the head, a doctor says that Reece and his fallen men all have the same brain sickness.
Back in the United States, Reece's life falls apart. Boozer is found dead. Reece sees signs that it was not a suicide. Soon, Reece returns home to find his wife, Lauren, and daughter, Lucy, dead in a hit made to look like street crime.
Reece lives through another try on his life at a clinic. He knows a group wants him dead, so he hides. He gets help from Katie Buranek and his friend Ben Edwards. They find a tie between the ambush, the killings, and a firm called Capstone Capital. The firm, led by Steve Horn, ran illegal drug tests on Reece's men using a medicine called RD4895. When the drug caused the sickness, the group set up the ambush and the killings to hide the failed test. They want to keep billions in cash and help the goals of Lorraine Hartley.
Reece makes a list of names. With help from Marco del Toro, he goes to Mexico and hits a safe house. Reece kills the men who murdered his family and learns they were hired guns. Back in the U.S., Reece goes after the group. He enters a mosque to kill an Imam who helped the ambush, cuts off his head, and leaves it on the gate. He breaks into the home of Josh Holder, an agent who helped hide the truth, and kills him after taking files from his computer. Reece finds Saul Agnon at a resort and hurts him to get the truth about Admiral Pilsner and the Hartleys. He then kills Agnon by making it look like a drug overdose.
Reece finds Mike Tedesco and forces him to wear a bomb into Admiral Pilsner's office. Reece blows the bomb from far away, killing both men. Reece goes to the Florida Keys to find Captain Howard. He catches Howard and leaves him to die in the swamp. Meanwhile, the Secretary of Defense starts a hunt, calling Reece a terrorist.
Reece goes to New Hampshire and misses a trap set by federal agents. He lets the leader live before leaving. With help from Liz Riley, Reece moves to New York City and hits J. D. Hartley, using a bomb to break his armored car. The path goes to Fishers Island, where Lorraine Hartley and Steve Horn hide. Reece gets inside and kills the guards. Inside, he sees Hartley and Horn, but finds Ben Edwards there too.
Edwards says he was with the group from the start, selling out Reece and his men for cash. He has a remote for bombs on Katie Buranek. Horn tries to talk Reece into killing himself to save Katie. Instead, Reece kills Horn and Lorraine Hartley. He then shoots and kills Edwards, freeing Katie. They burn the house to hide the proof and leave.
Reece puts Katie on a plane with the proof, but he stays behind. He thinks he is dying from the sickness. He gets on a boat and heads out to sea, crossing off the last names as he goes. His doctor leaves a message saying the growth is not deadly and he could live with a medical fix. By the end, Reece stays at sea, not knowing the news, thinking he is heading toward his end.
