Coast of Gaul, AD43. Roman commander and all-round hard bastard Aulus Plautius (David Morrissey) surveys his prisoners, an animal carcass draped fetchingly across his shoulders. Ninety years after Julius Caesar pitched up on its shores, only to meet the Druids and leg it, the Romans are finally ready to conquer the Celts. But just as the ships are set to sail, four soldiers are caught deserting.

“Britannia is a cursed land, ruled by the dead,” gibbers one. “They feast on human flesh,” whimpers another. To be fair, they’re not too wide of the mark. While our current leaders hopefully stop short of dining on the innards of benefits claimants, were the ancient Romans to clock the suited corpses that currently line Parliament, and the primeval acts of self-immolation occurring therein, they’d be back on their ships before you could say “Hail Caesar”.

In Jez Butterworth’s Britannia (Thursday, 9pm, Sky Atlantic), the Romans find our sceptred isle awash with mildewed mystics and festering yokels engaging in surreal fertility rituals among the standing stones. Even Plautius’s right-hand man, Vespasian, is struggling to see the appeal. “What do they have up here, besides trees and nightmares?” he asks, daydreaming about cocktails on the Amalfi coast.

Offering persuasive evidence that commissioning editors will greenlight anything involving sandals, swords and ladies in leather bustiers, the show arrives hot on the heels of last year’s gleeful trash-fest Bromans and the beardy extravaganza Vikings. More pointedly, it is hoped that this epic will plug the gap left by Game of Thrones. But compared to this bunch, the inhabitants of Westeros make for gentle teatime drama. What Britannia lacks in sibling sex and skeleton armies, it makes up for in plain weirdness. Savagery, superstition and strange visions abound. Not for nothing has its director suggested watching it after a massive spliff; Britannia is completely off its tits.

There’s the theme tune that eschews orchestral bluster in favour of Donovan’s Hurdy Gurdy Man, and leading characters such as the outcast Divis (Nikolaj Lie Kaas), all minging teeth and mouldering complexion, who pride themselves on the smell of death emanating from their underpants. Also among Britannia’s parade of misfits and weirdos, we find actual, three-dimensional women, among them Kerra (Kelly Reilly), awesome warrior and thoroughly narked princess, and Zoë Wanamaker’s leader of the Regni tribe, given a Mad Max makeover and the excellent gambit: “I shit on the souls of your dead.”

Elsewhere, there’s Kerra’s father, King Pellenor (Ian McDiarmid), chief of the Cantii, who greets his visitors with a gracious: “Fuck off back to Rome”, and head Druid Veran, unsettlingly played by Mackenzie Crook as a merging of Gollum and Lord Voldemort. An encounter with Veran sends mortals out of their minds, even without herbal assistance.

This is not to say that Britannia doesn’t cleave occasionally to ye olde traditions of histoporn: nothing says costume drama like a topless woman bouncing cheerfully atop her second husband as her first one looks on. Furthermore, it couldn’t give a flying one for historical accuracy; the script would give Mary Beard a hernia. But all’s fair in love and polytheistic pagan ritual. Britannia will mess with your head. Roll a fat one and enjoy the ride.