"It only took one second. One second, one cat, one swerve - and everything changed." Osamu Miya is driving home after a long shift at his beloved onigiri shop when he answers a call from his twin brother, Atsumu. It's supposed to be a routine check-in - teasing, warmth, nothing unusual. But when a cat darts across the road and Osamu swerves to avoid it, he loses control of the car and plunges into a rain-flooded ditch. Trapped. Alone. Slowly sinking. With water rising and air thinning, the only thing keeping Osamu grounded is Atsumu's voice through the phone. And Atsumu refuses to hang up. In this emotional rollercoaster of grief, guilt, and twin-bonded love, Stay on the Line explores what it means to hold on - to hope, to each other, and to life - even when everything is slipping away. Osamu may drown. But he won't do it alone.

After an intense practice, Atsumu Miya's drive home turns into a nightmare when a catastrophic tunnel collapse traps him in his wrecked car. With crushed legs pinned under the dashboard and metal embedded in his chest, Atsumu faces a desperate fight for survival-physically and mentally-deep underground, isolated and fading fast. Meanwhile, Osamu Miya, still healing from his own near-death experience, receives Atsumu's sporadic calls. Though helpless to reach him, Osamu becomes Atsumu's lifeline, offering strength, comfort, and hope through every trembling word over the phone. For 15 harrowing days, trapped and fading, Atsumu clings to Osamu's voice-until rescue arrives, but the road ahead is just beginning.