Tom Bower is an assassin among political biographers. When children hear that daddy has come to Mr Bower’s attention, they weep all the way to school and fail their exams. Wives change the locks or hire a divorce lawyer. Bower’s books hiss with menace and attract writs. Yet this one gives ‘unauthorised biography’ a bad name.

As a rule, Bower is keen to spray around blame. Here, however, we find him firing blanks. Might this have something to do with the peerage recently given by Boris Johnson to the former Evening Standard editor Veronica Wadley? In Bower’s account, it was Wadley who persuaded Boris – he is never ‘Johnson’ here – to run against Ken Livingstone in the 2008 London mayoral contest, and she backed him through thin and thinner thereafter. Bower just happens to be married to Wadley (now Lady Fleet), a fact he discloses to innocent readers only in the acknowledgements on page 527. His reticence is telling and reflects the selective nature of Bower’s scorn, from which he gallantly shields Boris – who is, he coyly admits, ‘not a stranger in my home’.

Bower’s intentions are not immediately clear as he relates the now-familiar story of the four older Johnson siblings’ savagely dysfunctional upbringing. Bower has had access to Boris’s mother’s version of events, and here provides details of her husband Stanley’s regular physical and verbal assaults. This means that Boris’s father is cast as villain-in-chief here. We are invited to believe that his eldest son was burdened with awareness of his father’s ambitiously selfish, restless, priapic and mean but also frivolously self-defeating failings. Boris grew up a solitary, insecure child, determined to do much the same as his father but better.

It is a bleak but riveting story, involving ‘parentless’ children as feral as council estate waifs – and sometimes as hungry too. But it serves a useful purpose, allowing Bower to present Johnson as a victim, a clever ‘mixed-race outsider’ making his way among dim, snooty aristos and arrogant members of the liberal elite. Johnson strives against envious ‘Boris haters’ in the press, Parliament, the civil service and, of course, Brussels, where he first gained fame reporting on bent bananas and condom sizes.

How can Bower, the tenacious researcher, square the record of Johnson’s profligacy, opportunism and dishonour, much of which he reports in masochistic detail, with the generous and principled hero of his imagining? With evident difficulty, but he tries, employing the Boris-like device of blaming each successive disaster on someone else. ‘Had he sought to be better briefed’ is the kind of weasel wording deployed here to exculpate Johnson.

There are few jokes in a Bower biography but one laugh-aloud moment in this one comes when George Eustice, David Cameron’s press spokesman while in opposition, leaves a message on Johnson’s phone. Before hanging up, he says to a colleague next to him, ‘That man is a complete cunt’. Aggro-averse Boris rings back in a spirit of mollification and Eustice lives to tell the tale. Brexit-compliant by conviction, not opportunism, he is now one of Johnson’s Cabinet ministers. Most of those who cross Boris are not so lucky.

The same goes for Bower. So Jeremy Corbyn is an ‘anti-Semitic Marxist’ and Keir Starmer is ‘not the finest legal brain at the Bar’. Michael Howard is ‘impetuous and blinkered’, Philip Hammond is a ‘class-conscious’ transport secretary who does ‘dry freakery’, David Cameron has ‘drunk his own Kool-Aid’ and Chris Patten was a ‘presumptuous chairman’ of the BBC. Simon McDonald, permanent undersecretary at the Foreign Office (until he was effectively sacked by Johnson) is ‘unctuous and fluent’ – and also ‘from Salford’. The Department for Transport is staffed by ‘notoriously incompetent civil servants’. The BBC and The Guardian (even its investigative scoops) are relentlessly derided. On the other hand, Jacob Rees-Mogg is ‘an exceptionally polite and well-educated fund manager’ and the Tory peer Greville Howard’s house has ‘elegant’ flower boxes.

This would matter less if Bower’s partisan narrative was softened by witty or worldly political judgements. Instead, they are harsh and crass. At one point he describes populism as ‘the sin of serving the needs of the working class while ignoring the self-interest of the liberal elites’. Billionaire outsider Donald Trump could not have put it better. Bower believes that Johnson really has ‘got Brexit done’, though he parks that unfolding story with Johnson’s December election victory, shifting the focus to the coronavirus outbreak.

The pandemic is a rare example of a Johnson career crisis – political, financial, sexual, administrative – that has not been self-inflicted. But in largely ignoring it for two crucial months, and then by dithering, Johnson has made it his own. Bower blames the medics and scientists and useless health officials for Britain’s high mortality rate (and foreigners for fiddling their own figures). There is some truth in his strictures, but no one has recently called Chris Whitty or Patrick Vallance a ‘genius’, as Charles (coincidentally, now Lord) Moore did of Johnson. The pandemic presented Johnson with the prospect of a Churchillian moment and he flunked it. Careless, clinically obese and self-deceiving, he caught the virus himself and nearly died while his revolutionary Dr Strangelove was taking an eye test at Barnard Castle.

The phrase that often springs to mind here is ‘opportunity cost’. Back in January, Johnson was too busy with the Brexit bongs to fret about some Chinese clap. A leader whose aides fear to bring him bad news, is careless with details and facts, is prey to his own demons and is constantly juggling the Petsys and Carries is unlikely to be able to master a real challenge. ‘Be careful what you wish for’ might serve as an epitaph for World King Boris I, if he gets one.

From Eton, through Balliol and Brussels, sort-of editing The Spectator and pretending to be foreign secretary, Johnson mostly got away with it, leaving family and friends, colleagues and staff to sweep up the wreckage. Disdain for rules and hard work coupled with impudent charm got him through. Bower is just another enabler. By the time of the 2019 election, ‘the majority of the British public accepted he was a rogue and wanted to move on,’ he writes. But Johnson’s bluff has now been called and Bower comes close to admitting it in his epilogue.

Would this have happened if Labour activists had not stood by the hopeless Jeremy Corbyn, allowing their Tory counterparts to safely (as they thought) indulge their own reckless fantasy of a Johnson leadership? Bower does not ask. But Corbyn still has his allotment. What Bower brings sharply into focus here is how lonely Johnson is, how dependent on excitement and applause to stave off recurring depression and sustain his erection. Being PM was a form of 24/7 therapy until the coronavirus burst in. As Johnson might put it in one of those £5-a-word Daily Telegraph columns, he is not so much Caligula as the horse.