Wanted: attractive young girl (boy may be considered). Must have strong ankles, healthy lungs, and no family ties. Ability to say 'what's that, Doctor?' in many different ways a necessity. Bizarre taste in clothes may be an advantage. Must be able to deal with traumatic situations involving life and death with no obvious after effects.
They didn't always ask to travel with the Doctor. And even if they did, they didn't know what the consequences would be.
Were Ian's travels foretold? What paradoxical conundrums faced the Doctor, Charley and Will Shakespeare in Ancient Troy? And just how difficult is it to get a job when you can't account for a gap of several years on your CV?