In a post-apocalyptic world, a young boy named Johnny discovers a path leading to a secluded ravine. During a family picnic, Johnny encounters an astronaut and wanders into a cave where he's knocked unconscious by an explosion. When he awakens, he finds himself in a desolate world dominated by alien invader Ro-Man, a creature resembling a gorilla wearing a space helmet. Ro-Man has been sent by the Great Guidance, the leader from his planet, to eradicate all human life on Earth using the powerful Calcinator Death Ray. Despite their attempts to survive and adapt to the hostile environment, Johnny and his family must evade Ro-Man to stay alive.

As Ro-Man launches his merciless campaign, Johnny's family strives to elude capture while seeking ways to counter the alien threat. The survivors include Johnny's sister Alice, who ingeniously tries to interfere with Ro-Man's plans, and their scientist father, who hopes to find a weakness in Ro-Man's technology. As Ro-Man hunts them relentlessly, he begins developing feelings for Alice, causing inner conflict and tension between his mission and emerging emotions. Meanwhile, the Great Guidance pressures Ro-Man to complete his genocidal task. Danger intensifies as both sides confront their fears and fight for survival, with the fate of humanity hanging in the balance.