Thomas Jerome Newton is a humanoid alien who arrives on Earth from a distant planet, landing in the stark landscapes of New Mexico. To the world, he appears as an ordinary Englishman, but he is here on a critical mission: to transport water back to his home planet, which is enduring a dire drought. Using the advanced technology from his homeland, Newton quickly patents several inventions on Earth, amassing considerable wealth through his role as the head of the World Enterprises Corporation, aided by his skilled patent attorney, Oliver Farnsworth.
During one of his visits to New Mexico, Newton crosses paths with Mary-Lou, a lonely young woman from Oklahoma. Mary-Lou juggles numerous part-time jobs at a local hotel to make ends meet. As she introduces Newton to various earthly customs—ranging from attending church services to indulging in alcohol and exploring intimate relationships—the two form a bond and move into a newly built house near the site of his arrival.
Dr. Nathan Bryce, a former college professor with a questionable past regarding his female students, secures a position as a fuel technician at World Enterprises. He gradually becomes Newton's confidant, sensing that there is something decidedly alien about him. In curiosity, Bryce arranges a clandestine meeting where he uses a special X-ray camera to uncover Newton's true alien physiology. As Newton struggles with his growing addiction to alcohol and the distractions of television—often watching multiple screens simultaneously—tensions rise between him and Mary-Lou. In a moment of truth, Newton reveals his alien form to her; however, she is horrified and flees in panic.
As Newton completes his spaceship, ready for its maiden voyage to transport water home, he faces unexpected turmoil. Just before take-off, he is captured by the government and a rival company, leaving his fate hanging in the balance. In a shocking turn of events, his business partner Farnsworth is murdered, and Newton is confined to a locked luxury apartment deep within a hotel complex, where he faces rigorous medical examinations. The government keeps him sedated with alcohol, leading to the unfortunate fusion of his contact lenses with his eyes during an X-ray procedure.
Years pass within his derelict confinement, but Mary-Lou returns, now aged and affected by the ravages of time and alcohol. Their reunion is marked by an unsettling mixture of mock-violence and nostalgic intimacy, and they engage in playful antics involving toy guns and drinking games. Yet, their love has waned; Mary-Lou admits she no longer loves him, and Newton mirrors her sentiments. Eventually, the shackles of his restrained existence break down when he discovers that his prison is actually unlocked and makes the choice to leave.
With no way to return home, a disenchanted and alcoholic Newton records a message filled with alien signals, aspiring to send it back to his native planet. Meanwhile, Dr. Bryce, who has gone on to marry Mary-Lou, encounters Newton once more at a restaurant. Although Newton maintains his youthful appearance and vast wealth despite the passage of time, he is dripping in sadness and alcoholism. As he sits at the restaurant's outdoor patio, a poignant moment unfolds when he asks about Mary-Lou before succumbing to a drunken stupor, encapsulating his tragic journey on Earth.
