Twenty-five years ago, with Richter's Syndrome running rampant throughout the galaxy, the brilliant biochemist Nyssa, formerly of Traken, bade a painful farewell to her young family... and set off into the space, in search of a cure for this deadly disease. She never returned. Now, her grown-up son continues her work on the penal colony of Valderon, still desperate to make the breakthrough that eluded his presumed-dead mother. So when the TARDIS lands on Valderon, bringing the Doctor, Tegan, Turlough and Nyssa to its fortress prison, the scene is set for a painful reunion... but not only for Nyssa. The Doctor's past is about to catch up with him too...