Unfortunately, I missed the book before this one, The Monk Who Vanished, because I have the feeling something happened in it that was important to the Sister Fidelma series. This is the ninth book in the series and we find that Sister Fidelma, sister to the King of Cashel, a religieuse of the Celtic Church, and a dalaigh, or officer of the court (and the second highest degree you can get) is on board the ship, The Barnacle Goose, heading for what is modern-day Spain and the city of Iberia, where the Shrine of St. James is, in order to reflect upon her relationship with the Saxon monk, Eadulf, and the crisis of faith she is experiencing. Among the group of pilgrims, the leader, Canair, does not show up in the morning when the ship leaves, because, unbeknownst to some of the pilgrims, she has been murdered at the Inn in Ardmore.
Sister Muirgel takes over since she sees herself as the next one in line due to her nobility. When Fidelma comes aboard, the Captain, Murchad, recognizes her name, and therefore her rank and reputation as a dalaigh. Fidelma wants to just be a Sister on a pilgrimage and asks that he not mention any of this to anyone. Fidelma is to be bunked with Sister Muirgel, but when she arrives, the Sister is suffering horribly from seasickness and Fidelma gives in and takes a room all by herself. After a brutal storm their first night out, Muirgel is missing and a search of the ship fails to discover her, so it is assumed that she went overboard. Now Cian, a man that Fidelma had an affair with ten years ago when she was a student that ended with him marrying another woman, sees himself as the one to take over. Cian was a warrior in the High King's Army, until five years ago when an arrow that pierced his right arm, made it useless. He felt he had no choice in life but to become a religieuse, even though he does not have a religious bone in his body.
Sister Fidelma begins an investigation, at the behest of the Captain, to uncover what happened to Sister Muirgel. Wenbrit, a sailor on the ship, finds Sister Muirgel's robe with a cut in it and blood on it, which makes no sense if she went overboard. Later, Sister Muirgel shows up in her room, dying from a knife wound, holding Sister Canair's crucifix in her hand, which she gives to Sister Fidelma. Brother Guss claims that the two were in love and that they had been in the room next door to Canair's at the Inn and heard her die. After this happened, Guss tells Fidelma that Muirgel was terrified for her life, which is why she faked her death.
These are only two of the deaths that occur on this cursed ship that also endures being chased by Saxon pirates, fearful storms, and watching a ship crash on the rocks and only being able to save three of the men on board. One of the men is Toca Nia who accuses Cian of some horrid war crimes. After he dies, and Cian disappears, suspicion begins to fall on Cian, who also had love affairs with the two other dead women (but then Cian seems to have slept with most of the Sisters at the Abbey). Fidelma really wishes that her friend Eadulf were there. He is a Watson to her Holmes; a Captain Hastings to her Hercule Poirot. He always notices something she misses or says something that just makes everything click into place. This ship is not just one of death, but also of lust, as they all seem to have slept with each other. Fidelma really struggles with this one. She cannot seem to see the path to the truth and when attempts are made on her life, she realizes that she is dealing with a dangerous and possibly insane killer who has developed a taste for murder and will not stop unless Fidelma can uncover their identity in time.
Link to Amazon: https://www.amazon.com/Act-Mercy-Celtic-Mystery-Fidelma-ebook/dp/B006CQ99LC/ref=sr_1_2?ie=UTF8&qid=1510584608&sr=8-2&keywords=act+of+mercy
To be enemies means some feeling remains between us. There is nothing between us now. Not even bitterness.
--Peter Tremayne (Act of Mercy p 106)
It seemed to her that seamanship was nothing but long, boring periods of inactivity, interspersed by frenetic outbursts of action and turmoil.
--Peter Tremayne (Act of Mercy p 124)
Yet, madness can be a gift from God, so perhaps she is blessed.
--Peter Tremayne (Act of Mercy 171)