The Doomsday Machine

A spy discovers that the Chinese government has created a doomsday device (the "key" to which, "only Chairman Mao has") capable of destroying the Earth. Half of the male flight crew are replaced by women shortly before take-off, including one Russian. After leaving Earth, the crew 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.