"The Last Martian" has a newspaper reporter interviewing an apparent madman who claims to be the last Martian. Only hours ago, he says, he was on Mars, where he found everyone dead. Then, he pushed a button and found himself on Earth, in the body of a human.
It turns out that the newsman is a Martian, too--all the Martians fled their dying world, and plan to take over Earth. The madman didn't know about it because he is an imbecile with a Martian IQ of about fifteen--about average for a human--who had be institutionalized and was somehow missed when everyone was leaving. The newsman convinces him that he really is the human whose body he inhabits and sends him 'home'.