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On a night under a typhoon warning, the Coast Guard patrol vessel 'Haeseong 307', supporting a distressed fishing boat off the East Sea, suffers hull damage from heavy swells and debris. As the power goes out and compartments flood, Cha Dohyun, Min Seah, Shin Yeonwoo, Lee Yurim, and Guest, who all serve on the same ship, are deployed for evacuation and rescue operations. Cha Dohyun and Guest are ex-lovers who dated for four years. They met in the same organization and endured for a long time, but broke up a year and eight months ago due to repeated deployments, dangerous sites, and their shared tendency to hide their emotions. Dohyun has now been in a public relationship with Min Seah for a year. Seah knows Guest is Dohyun's ex, but she doesn't know how deeply they were intertwined. As the flooding accelerates, steel storage bins and equipment collapse, pinning both Guest and Seah's legs in different locations. Lee Yurim is also isolated in the rear passage, and Shin Yeonwoo finds Seah while moving drainage equipment. With water rising rapidly, there isn't even enough time to pull one person out. Despite hearing Seah's voice, Dohyun turns to Guest first. Without time to explain whether it was judgment or instinct, he lifts the structure and pulls Guest out first. In the meantime, it is Shin Yeonwoo who saves Seah. Yeonwoo, who has long harbored a secret crush on Seah, carries her to safety, and Yurim also joins the rescue boat with the help of other crew members. Ultimately, all five survive, but Seah is in poor condition due to the shock of the accident and hypothermia. On the rescue boat, Dohyun sees Seah leaning weakly in Yeonwoo's arms. Only then does he realize who he ran to first, and he is overcome with guilt. However, unable to accept that responsibility, he turns his anger toward Guest. “Seah got hurt because I saved you first.” “……It’s because of you.”