The Boy from Nowhere is not an easy book to explain, partly because its subject, the actor Gregor Fisher, does not find it easy to talk about who he is. “I don’t know if I know who I am or not,” he says at one point and that uncertainty, that wall of cautiousness, surrounds the whole book. Rab C Nesbitt, Fisher’s most famous character, is cocky, swaggering and bolshy; the actor who plays him is wary, watchful and shy.

The theory of the book, which has been written by the journalist and columnist Melanie Reid, is that Fisher’s cautious, uncertain nature can be attributed to his extraordinary upbringing. Fisher must be one of the few people in the country to have been adopted twice - the first time when his mother died of a heart condition, the second when his first adoptive mother died in a fire. He was then brought up by a third adoptive family, headed by a distant and unpleasant 1950s patriarch.

For most of his life, Fisher knew very little about of the details of these adoptions and Reid’s book is an attempt to unravel it all. Early on, the actor compares his life to a pile of spaghetti, or a tangled ball of wool, and Reid’s job is to rifle through it, sort it and lay it out for us in a readable and entertaining way.

She succeeds in doing so with an elegant, thoughtful writing style that it is not always a comfortable fit with Fisher’s blunter, gruffer ways (the book is mostly in Reid’s voice with occasional interjections by Fisher and conversations between the two of them). She also structures the book rather like an episode of the family history television programme Who Do You Think You Are?, climbing back up his family tree and stopping to tell some of the stories.

The details of the stories are interesting, partly because most Scots will recognise some elements in their own family historys: the lack of heating (“It was always cold in the 1950s”); households organised around the whims of the father (“This is my house”); and the gap between the rich and the poor (“Life taught them not to expect too much”).

The story of Fisher’s birth parents is also a reminder of what could happen if you were born illegitimate in the sanctimonious 50s. In one of the most moving sections of the books, Fisher says he can’t understand why he was abandoned by his father and Reid tries to comfort him. “He had no choice,” she says. “Not in those days.” He was a politician. A government officer. Respectable. His mother was an unmarried clerkess in a factory. As Reid puts it: a Nobody to her lover’s Somebody.

The Boy From Nowhere is an attempt to explain the consequences of that for Fisher, although it is rather hampered by his reticence and what feels like a reluctance to really talk about his emotions. This probably explains why the book has been structured in a slightly counter-intuitive way. The natural way for the story to have been told would have been in Fisher’s own words, but for whatever reason he prefers someone else to tell the story for him. This leaves Reid to come up with a compromise, which is that she does most of the story-telling with Fisher interjecting when he’s told to.

The result of this compromise is that Fisher emerges as a sympathetic, but frustrating character, which may be one of the few similarities between him and Nesbitt. Who couldn’t sympathise with the man who was told bluntly, in the days before counselling: “You’re adopted”? He also had to deal with the fact that his adoptive mother didn’t call him her son in his will but “the boy who lived with me”. Those kind of experiences leave scars.

What Reid spots, because she is an emotionally intelligent writer, is that one of the scars is Fisher’s wariness and caution. Rab C Nesbitt became famous for swaggering up to the camera and ramming his face into ours, sometimes so close you could smell the grease on his head bandage, but behind him, backing off, is Gregor Fisher. The problem this creates for The Boy From Nowhere is that you can’t back off and also expect to create an entirely engaging memoir. The only way to tell the story of your life is up-front and up-close.