The overarching narrative reveals that alien Do Min-joon originally missed his departure in 1609 after rescuing young widow Yi-hwa, who ultimately sacrificed her life to protect him. This left Min-joon with 400 years of unresolved trauma. In 2013, with three months left before his comet returns, he meets Cheon Song-yi, who carries Yi-hwa's appearance and spirit.
The suspense arc culminates as chaebol heir Lee Jae-kyung—who murdered pregnant actress Han Yoo-ra and institutionalized his ex-wife—is brought down through a wiretap operation led by Hwi-kyung and Prosecutor Yoo, receiving a life sentence.
The biological stakes dictate that if Min-joon stays on Earth past the comet's window, his body will disintegrate at a cellular level. During the Episode 21 meteor shower, he dissolves in front of Song-yi and becomes trapped in a 3D space wormhole. Over three years, Min-joon learns to project himself back to Earth, expanding his visits from seconds to over a year, achieving a sustainable, enduring romance defined by continual effort.