Ricky Gervais has always made it his business to say the unsayable. But he pushes his luck in Science, his latest and weakest stand-up show. His three earlier efforts danced that fine line between arrogance and mock-arrogance, yet largely came up funny and fresh. Here, he aims for too many easy targets. It feels cheap.

And it all starts so promisingly. So expensively, too. The pre-show videos are all plugs, whether for charities, Gervais’s two new films or this Sunday’s The Office retrospective on BBC2. But they’re sharp and silly enough to make an art form out of ironic self-promotion all over again. And when the curtains part to reveal a Frankenstein set, with brains in bubbling vats, it makes the rest of the Edinburgh Fringe look like amateur hour.

Yet Gervais throws away his advantage with a humdrum opening routine about Britain’s Got Talent. He goes on to jokingly insult fat people, travellers, ugly autograph hunters and a mentally impaired Ken Dodd fan.

However ironic his intention, the gags aren’t sharp enough to stop this from feeling mean-spirited. He gets more blatantly self-satirising - “I’m all for ending famine, as long as it doesn’t affect me in the slightest”. Yet too often the gap between what Gervais says and what he means is something we have to take on trust.

So the context isn’t there to sell his rape gag, his gay gags. He brings out a childhood picture book about Noah’s Ark, then picks holes in its simplistic assumptions. It’s fish-in-a-barrel time. Stewart Lee and Richard Herring did this kind of thing a decade ago, and did it better and with a more palpable moral sense. If the point is that such stories pollute young minds, it’s not well made.

Gervais remains a charismatic performer. There are some fine lines, some engaging physical comedy. The crowd award him a big ovation. Yet his biggest laughs come from mocking references to his podcast sidekick Karl Pilkington. The tone is troublesome, muddied still further when he softens the overweening persona for a nice pair of stories about a jogging mishap and a dinner-party faux pas.

We know Gervais means no harm. We know that because, well, he’s a brilliant man. And because he tells us so at the end in what amounts to a one-man post-match analysis. “The thing about off-colour jokes,” he explains, “is that we tell them to people who know we’re not really like that.” But when the bulk of your material points one way, you need to do more than say “only joking” to subvert it. Gervais argues that he can justify all his jokes. I would argue that he can’t grasp their cumulative effect. Science is “an investigation into the rational and the non-rational,” he suggests. Which is a pretty fancy way of defining comedy for your audience. Here, at least, the grandiloquence is for real.

Tour begins October 6 at the Portsmouth Guildhall, and runs to Dec 16. www.rickygervais.com