Here Be Dragons is the first of Penman's trilogy about the medieval princes of Gwynedd and the monarchs of England. England's King John uses his out-of-wedlock daughter Joanna as a negotiating tool by marrying her to the Welsh king Llewelyn to avoid war between England and Wales.
Joanna and Llewelyn's marriage is marred by resentment from Llewelyn's illegitimate son, Gruffydd. Joanna gives birth to two legitimate children, Elen and Dafydd. Growing animosity between the English and Welsh results in Joanna having to act as a diplomatic intermediary between her husband and her father, and the situation deteriorates when Gruffydd is taken hostage by John and narrowly escapes execution.
Joanna becomes determined that her own son, Dafydd, will be his father's heir as ruler of Gwynedd, disregarding the Welsh law that all sons should receive equal shares of their father's inheritance. Family disagreements lead Joanna into an affair with William de Braose, who is several years her junior and whom she has met earlier in the story when he was a hostage in Llewelyn's household.
Their affair is discovered and William is executed. Joanna is placed in secluded captivity, but at the end of the book Llewelyn comes to find her and offers her forgiveness.