A vacation planet, Tranquamilia, suffers from a mind-control device broadcasting rage, turning its populace into aggressive beasts. With the TARDIS working perfectly, the Doctor and Peri decide to take a holiday. But where? A long-forgotten piece of equipment in the TARDIS storage locker sends them to the peaceful and idyllic continent of Armelia - home of the Doctor’s old friend Ravlos. But the land where they emerge is far from peaceful. A hate ray is regularly sweeping Armelia, turning its inhabitants into savage beasts, and there is only one place it can originate - the planet Tranquamilia's other continent, home of Armeila's old enemies, the Tranquillons. Or is that the only place? Because somewhere far above the planet events are watched by the slimy super-salesman, Mordant, who has his own unscrupulous plans.
