The story is that Hitler claimed to Neville Chamberlain when they met in Munich, that during World War I he had been part of a German unit under fire from British soldiers, and while the Germans, including Hitler fell back, a British soldier could have shot him, but did not. And Hitler had a copy of a painting by Matania hanging in a room where they met depiciting the Green Howards (a British unit) in combat, and pointed to a soldier in the front, claiming that was the man who spared his life. That man just so happened to be Henry Tandey VC, the most decorated British private soldier of the war.
As it turns out, the story is probably completely bunk. Hitler generally seems to be referring to the battle during which Tandey received the Victoria Cross, among other honors (in fact, he got a certificate explaining he wouldn't receive any further medals for bravery because there weren't any left to give him), but a) that battle was in 1918, and the painting is of a battle from 1914, b) as far as Johnson can discern Hitler probably wasn't even with his unit at the time of that battle, and if he was, they were 50 miles away, and c) Hitler was a regimental dispatch runner and wouldn't have been close to the front lines anyway. Oh, and the version of Tandey in the painting apparently bears no resemblance to the actual person.
Johnson saves the discussion of this legend, the theories on how and why it came about, and how Tandey responded to it (mostly dismissing it by remarking he didn't remember seeing anyone like Hitler, though his response varied some over the decades) for the end of the book. The first three-quarters are devoted to the rest of Tandey's life, as best as Johnson can reconstruct it. He doesn't have much to go on, as Tandey was fairly private, and only sporadically close to his family. It's especially notable in the chapter on World War I, as Johnson has no diary or letters home to work from, only a couple of times where Tandey has written something or been interviewed that was featured in a newspaper. Beyond that, he discusses the general life of frontline infantry during the war, and draws inferences from there. Which seems a little dodgy, when he's quoting from other soldiers letters home as to how soldiers felt about the possibility of death, and then musing on Tandey's based on this.
He does discuss Hitler's upbringing a bit, as a compare/contrast to Tandey's, which he continues through the Great War. What's interesting in that part, though, is where he points out the revisions Hitler made to his wartime experiences, versus what the surviving records suggest is more accurate. this also helps establish a basic timeline where he can look for other points in the war when the two could have possibly crossed paths so Tandey could unwittingly spare him.
Ultimately, I think the fact Tandey is such a private person works heavily against the book. There isn't much to work with, because he isn't the kind of person to write volumes of correspondence (or the people he sent it to didn't care enough to keep it), so a lot of the book is guesswork and supposition. Which makes it harder to be drawn into the sections about Tandey's life, which made it harder to stick around until Johnson got to the hook of the book. Fortunately it's only about 170 pages, so you can still reach that section quickly enough.
'Henry may also have resorted to another much-used method of lice control, involving running his fingernail through the seams of his clothing. The lice were referred to as 'chats' and their removal became known as 'chatting' - the men would sit together 'chatting'.'