A little boy goes missing; a policewoman begins drawing cave paintings; and the employees at the mysterious Glasshouse are desperate to keep everyone away — the Doctor suspects it's all down to a group of homo reptilia. His assistant, Liz Shaw, has ideas of her own and has teamed up with a journalist to search for people who don't exist. While the Brigadier has to cope with UNIT funding, the breakdown of his marriage and Geneva's threats to replace him, the Doctor must find the reptiles alone.