Kim Jang-ok and Joo Dong-sung, both members of the Korean independence group Heroic Corps, approach the affluent Kim Hwang-sub, ostensibly to secure military funding. However, this meeting turns out to be a trap laid by the Japanese forces. In a fierce gun battle, Kim Jang-ok fights valiantly but is ultimately faced with a heart-wrenching situation when his old friend, Lee Jung-chool, now an officer with the Japanese police, confronts him. To escape the turmoil, Kim takes his own life. Meanwhile, Lee chooses to discreetly release Joo Dong-sung, which raises eyebrows within the ranks of Heroic Corps. Tensions escalate as Jo Hwe-ryung accuses Joo of being a traitor, leading to Kim Woo-jin's intervention, which results in Joo's banishment due to his compromised identity.
In the aftermath, the Japanese authorities assign Lee Jung-chool to investigate Kim Woo-jin. Despite their shared past and recognizing each other's true roles, both men opt to maintain a facade of friendship. Lee's new partner, Hashimoto, is actually tasked with surveilling him. As the pressure mounts, Lee tries to protect the Heroic Corps by delaying Hashimoto's efforts, but they only narrowly manage to escape to Shanghai. There, amidst the tensions, Heroic Corps leader Jung Chae-san and Kim Woo-jin persuade Lee to join their efforts. Under growing pressure, Lee ultimately shifts his allegiance, agreeing to help transport explosives back to Korea.
On Korean soil, Lee cautions Kim Woo-jin that a spy lurks within their ranks. To unmask the mole, Kim concocts a plan involving varied rendezvous times for each member of the group. The ploy works, revealing Jo Hwe-ryung as the traitor. In a dramatic encounter aboard a train, Kim executes Jo in the name of Heroic Corps, emphasizing the group's commitment to their cause. However, upon their arrival in Gyeongseong, chaos ensues when police confront them. In the midst of it, Yeon Gye-soon bravely kills officers who attempt to arrest her, igniting a violent shootout that results in numerous casualties, including many resistance members. Kim, however, manages to evade capture and successfully smuggles the explosives into Korea.
As scrutiny falls on Lee, he is commanded to locate Kim and resorts to torturing Gye-soon to keep his cover intact. However, he falls into a trap set by Joo Dong-sung, leading the authorities directly to Kim's hideout. Following a desperate attempt to escape, Kim is apprehended. In a desperate bid to keep his secrets safe during interrogation, he bites off his own tongue. Lee also finds himself imprisoned but pretends to remain loyal to the Japanese authorities to obtain a lighter sentence. During the trial, he emotionally declares that he was never with the resistance, adhering to Kim’s earlier request to pose as a non-ally so their mission could persevere.
Once released, Lee learns of Gye-soon’s tragic demise and hears about a gathering hosted by Japanese collaborators. Holding onto Kim’s last wish that at least one individual survive to finish their quest, Lee strategically plants explosives at the party. Concurrently, a Heroic Corps operative carries out the assassination of Joo Dong-sung. When the bomb detonates, it claims the lives of several high-ranking officials, including Higashi. Back in prison, a mute Kim Woo-jin gains knowledge of the successful mission, causing him to smile with relief, knowing that his sacrifices held significance. Eventually, Lee confronts Kim Hwang-sub, whose betrayal was responsible for Kim Jang-ok’s death, and takes vengeance by killing him, honoring his fallen friend's memory.
