A spy (Essie Lin Chia (fr)) discovers that the Chinese government has created a doomsday device (the “key” to which, “only Chairman Mao has”) capable of destroying the Earth and it will be activated in 72 hours. Soon after, Astra – a two-year return mission to Venus by the United States Space Program – has its time of launch speeded up and half of the male flight crew are replaced by women shortly before take-off, including one Russian. Shortly before blastoff military alerts are put into effect.
After leaving Earth, the seven crew members of Astra deduce that they have been put together to restart the human race should the Chinese activate their device. Shortly after this, the device goes off and Earth is destroyed.
As Astra continues to Venus, the crew realizes that a safe landing on Venus is impossible unless the crew is reduced to three. One of the crew members tries to rape another, at which point she accidentally gets them both blown out of an airlock.
Two more crew-members – an American astronaut and the female Russian – are lost as they head out to repair a fault with the spaceship. However, they notice another spacecraft nearby and jump to it. The second craft proves to be a lost Soviet ship that disappeared piloted by a close friend of the Russian crew member. Though its pilot is dead, the astronauts successfully power up the Soviet ship. Before the two ships can rendezvous, contact with Astra is lost.
A disembodied voice cuts in, claiming to be the collective consciousness of the Venusian population. The voice informs the survivors (whom the Venusians refer to as the “Last of Man”) in the Russian ship that Astra no longer exists (presumably destroyed somehow by the telepathic Venusians), and that no humans (due to their “self-destructive” potential) will be allowed to reach Venus. It gives a cryptic message regarding the prospect of starting new life in a “very strange and very great” place, somewhere far “beyond the rim of the universe”, before the movie abruptly concludes.
By Directed by w:Herbert J. Leder, Harry Hope, and w:c:Lee Sholem (https://archive.org/details/DoomsdayMachine1972) [CC0], via Wikimedia Commons