The good news is that with Adriano Shaplin’s Tragedy of Thomas Hobbes the RSC launches a new-play season in the dilapidated but beautiful theatre that calls itself Wilton’s. Equally good is that, to judge by a play that opens with a sneering Cromwell ordering a Leveller to be “broken into pieces”, ends with Charles II watching a comedy mocking his newly formed Royal Society, and in between involves battles within the era’s intellectual establishments, there’s to be no playing safe at this address. But the less good news is that the evening is an awful slog: not nasty, not brutish, but emphatically not short either.

I don’t think Hobbes’s famous dictum about life surfaces in the weird mix of the hifalutin and colloquial that passes for dialogue here. But especially at the start, when a dramatist should be providing you with a compass, you’re struggling so hard to identify the characters and decode a clotted, over-elaborate plot you inevitably miss this and that. What’s certain, though, is that Hobbes’s philosophy is never adequately outlined and, even with the estimable Stephen Boxer in the role, he isn’t interesting as a man either.

The historical Hobbes was a mild person whose belief in the strong state and powerful ruler came largely from a horror of war and a hunger for peace. Here, he’s always ferociously embattled with the rationalist boffins who believe in human progress. And eventually he’s rejected, Falstaff-style, by the king he once tutored, while his triumphant foes fall out among themselves, with Jack Laskey’s Robert Hooke, whom we’ve met eagerly carving up a live dog and playing with an air pump, ending up a near-lunatic.

Shaplin’s text combines with Elizabeth Freestone’s production to transform the stage into a jungle gym, Charles into the precursor of a modern pop star and the play’s tone into an awkward mix of the serious and comic. Ordinary speech has lines such as, from a gender-bending Amanda Hadingue as the natural philosopher Robert Boyle: “We wish to transcend your teeming insect war and trade it for a bird’s casual congress with the air.” You long for clarity and simplicity.

Then in comes Isaac Newton in a preposterous black wig to declare that he knows more than his Cambridge lecturers. “And I’m just 18,” he primly adds, leaving me at least wishing an apple would fall on his head and knock out him — and the play.

Box office: 0844 8001118. To Dec 6 2008

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