When The Doctor and his newest companion, Rani Koothrappali, visit the planet of Gallifrey upon a summon from the Time Lords, they discover that not all is right. A cold is quickly spreading across the planet, and only Time Lords are prone to it - so it's up to Rani to set things right, as the Doctor too begins to feel the symptoms. Can she save the planet with only one medical scientist by her side in time?