The film depicts an extraordinary story that unfolds as the gates of time open between late Goryeo and the present in 2022, when aliens appear.

After a short prologue, the plot moves to the modern day and then back to fantasy Goryeo, though scenes from the two time periods are interspersed.

The alien's consciousness can be awakened under certain circumstances, such as breathing in haava (the atmosphere of their native planet) or being cut by Guard's knife-shaped power source.

Guard, Thunder, and Yi-an work together to kill the alien pilot and prevent the Controller from releasing the remaining haava on board the ship.

In a final battle, Guard is destroyed, Thunder (in SUV form) sinks to the bottom of a river, and the Controller is badly wounded and forced to abandon its host.

Despite his unserious demeanor, Mureuk appears to have incredible talent for magic and martial arts, with an especially remarkable ability to recover from serious injuries.

If either Yi-an or the aliens obtain the Divine Blade and Thunder's location, they can return to the future and either complete or stop the haava release.

Mureuk rescued Yi-an from the river and the Controller took him as its new host, causing him to lose his memories of the incident; the film ends on this revelation.

[22] Choi revealed that while writing the script, he had specific actors in mind, and to his surprise, all of his envisioned stars accepted their roles.

[37] Kim Soo-young in her review for Korea Economic Daily stated that despite it feels bit burdensome and awkward at the beginning to accept the vast worldview of two completely different times and spaces, each busy following the narrative within it, the story flows smoothly in the latter part, creating a strong sense of immersion.

[38] Jo Hyun-na of Cine21 wrote that the characters, settings and production designs such as space ships lacked novelty, but praised the performance of actors.

[39] Choi Young-joo writing for No Cut News praised the director for his ability to combine different genres calling it "a challenge that should be taken at least once" though it seems excessive at times.

[40] My Daily's Kim Nara wrote that despite having powerful cast, the film did not meet expectations of the audience, and the director Choi Dong-hoon's own distinct color which was shown in his previous works is rarely seen in Alienoid.

[3] The film became a box-office failure, due to high production cost which required about 7 million (South Korean) admissions to cross the break-even point.