 After disappearing without a trace and being given up for lost, a manned probe to Mars is observed returning to Earth but will not respond to any contact. After mission control risks an automated landing, they find that there are only two survivors, one critically injured and with some strange growth on his arm. Doctors turn to the other surviving astronaut, Iris Ryan, for answers, and discover the Martians  angry warning which, to paraphrase, would be, "You Earth kids stay out of my yard!"