Resident Alien follows an alien who crash-lands on Earth, murders a small-town doctor, and assumes his identity (Dr. Harry Vanderspeigle) to blend in while searching for his ship, but his mission to destroy humanity gets complicated by forming friendships, especially with Asta Twelvetrees, who sees his true form, and facing new alien threats, leading him to question his mission and protect the planet instead, all culminating in a final season where he embraces his human connections and battles a new alien enemy to save Earth.