Fanglith

When a pair of political refugees is stranded on the galaxy's most barbaric planet, their son and daughter must save them - both from their Thought-Police pursuers and the terrible warriors of 11th-century Earth. Aided only by their ship's computer and a telepathic wolf, their first task is to rescue their mother from a nunnery. The planet is primitive and incredibly violent. They don't know the language, customs, or taboos. The Thought-Police are hot on their trail. But they are smart and resourceful - and they do have an espwolf with them...