Lamb tells the story of a young priest, Brother Sebastian, who works in a the Borstal on the Atlantic coast of Ireland, referred as 'a finishing school for the sons of the Idle Poor' by its head, Brother Benedict. There the Brothers teach boys to conform in a harsh, uncompromising regime which Brother Sebastian, whose real name is Michael Lamb, finds deeply distasteful. The Brothers teach the boys ‘a little of God and a lot of fear.’
When his father dies, leaving him a small legacy, the tie which kept him at 'the Home' is gone and he decides to leave and take a bullied, unhappy boy with him. His decision is also affected by the fact that he has made a vow of poverty and Brother Benedict expects him to hand his inheritance over to the Brothers.
Michael has formed an attachment to a 12-year-old boy called Owen Kane. Owen (the Gaelic word for lamb) is the youngest boy there and has been in the Home for two years. Brother Benedict beats Owen for painting graffiti on the wall outside, because it ends with the word OK - Owen's initials – despite knowing that it wasn't actually Owen who did it. Owen comes from a broken family and a drunken, abusive father. Michael cannot see how he will survive there and wishes to give him his freedom.
He secretly leaves the school and takes Owen with him to London hoping to be the boy's saviour. They pass themselves off as father and son and move from hotel to hotel, having fun together. Michael lets Owen smoke, play on gaming machines and takes him to a football match to see his favourite team Arsenal play, but Owen, an epileptic, has a fit. They have to slip away from the medical centre before questions are asked.
Owen sometimes prattles on and on and sometimes just sits silently. Michael feels embarrassed during the silences and recognises that Owen controls the communication between them. As the days and weeks go by, Michael became more comfortable with the silences and they laugh a lot, the boy’s childish sense of humour being similar to Michael’s.
As his money dwindles and news of the kidnapping reaches the Irish community, with Owen's picture in the newspaper, Michael finds himself running out of ideas on how to save the boy's life. About to fly back to Ireland, they come across an ex-army man called Haddock who tells them about a nearby squat and says they can move in. Michael returns to the hotel to find Owen in floods of tears, thinking Michael has left him. In an emotional scene Michael tells Owen he loves him and man and boy hug and hold each other tight.
Michael gets a job, leaving Owen at the squat, but returns to find that Haddock, who he knows is gay, is in his dressing gown, has his arm around the boy’s shoulders and has been letting Owen smoke pot. Michael is worried Haddock may have molested the boy or will try to and decides they have to leave.
The grim denouement of the novel poses the question as to how far one would go to save a boy’s soul.
