Thirteen-year-old Catherine is growing up in 1290 England. Thus, she's already expected to act like an adult as her mother trains her to become a lady of the manor. But Catherine wants nothing to do with this, and she certainly doesn't want her blowhard father to marry her off to a rich, ugly, unpleasant baron. Catherine will concoct any plan and pull any prank to keep herself from this fate, sometimes with hilarious results. She tells us all about it, and Medieval life, in the diary she keeps at the behest of her beloved brother Edward, a monk at a nearby monastery.
Catherine is an intelligent, spunky character who's way ahead of her time. She struggles with things modern readers can relate to: obeying parents, accepting societal expectations, and crafting her own identity. Funnily enough, she wants to be anyone but herself; she considers becoming a nun, a juggler, a minstrel, and all manner of other occupations to avoid marriage. In the end though, she does learn her own value, and finds happiness in her position.
The Middle Ages was a rather gross and gritty time period, and the author stays true to that. Bodily functions and diseases are discussed, as is death by childbirth, sometimes in detail. Sexual interaction, even incest, is hinted at with less than discretion. Catherine and other characters swear. Catherine's father regularly abuses her physically; she and other characters see this as completely normal. These things said, Catherine, Called Birdy is a great intro to the Middle Ages for appropriate audiences.