A hawk snatches a dove out of the air, a snaggle-toothed dude looks trippy in a stone circle, centurions mutter about giant squid, villagers dance around bonfires huffing mystery powder – just the first few minutes of “Britannia”, a spectacularly weird new show on Sky Atlantic, are enough to make you feel like the drugs are starting to kick in. Created by Jez Butterworth, whose film credits range from the Roman-legion ham-fest “The Last Legion” to the Bond film “Spectre”, and whose play “The Ferryman” is currently drawing insane numbers in London, “Britannia” is a wigged-out, gruesome, funny and strangely haunting collision of fantasy, history, comedy and camp. Watching it is like entering a cinematic cheese dream, where John Boorman’s “Excalibur” gets dragged backwards through “Game of Thrones”, “Doctor Who” and “The Wicker Man”. It’s mad as a snake, and – assuming you’re prepared to smoke what it’s smoking – the best thing to come on TV in some time.

“Britannia” opens during the second Roman invasion of Britain in 43 AD , nearly a century after Julius Caesar invaded, encountered the Druids and – to quote the opening trail – “turned his armies around and went straight home”. In Butterworth’s cheerfully semi-historical vision, Britain is a land of warring tribes and disputatious locals, where the Canti, led by King Pellenor (Iain MacDiarmid), are about to conclude a marriage pact with their old enemies the Regni. Naturally, everything goes wrong. As heavily armed Roman troops flood up the beaches, led by the terrifying Aulus Plautius (David Morrissey, draped in a wolf pelt and built like a rugby forward, above), the prospective groom is slaughtered at the altar on the command of Queen Antedia (Zoë Wanamaker), who kicks off proceedings with a rousing shout of “Scum! Coward! I shit on the souls of your dead!”

Out of his skull Mackenzie Crook as Veran

It only gets stranger from there. “Britannia” profits greatly from the historical murk that surrounds early accounts of Britain, and particularly the Druids, about whom we know next to nothing. So Butterworth is at complete liberty to make them a terrifying bunch of superbeings into extreme piercings and body modification, led by the fearsome Veran (Mackenzie Crook). Emaciated, pointy-fanged, heavily scarred and tattooed and with nasty metal rings set in the ends of his fingers, Velen has an aesthetic that combines horror with the crustiness of a dreadlocked hippy, rather as though Nosferatu had finally decided to jack in the bloodsucking and buy a barge on the Avon.

If that makes “Britannia” sound ludicrous, it shouldn’t. As in Butterworth’s play “Jerusalem”, which found mythic resonance and ageless mysticism in the drunks and dropouts of a Wiltshire caravan park, the various influences on this show come together in a strange grandeur of expression. Absurdity and comedy (I particularly liked Nikolaj Lie Kaas as Divis, a wild shaman with the hypnotic gaze of Danny from “Withnail & I”, and a pet rock called Big Pebble) give way to moments of shivery splendour and ghostly waywardness: a man plummeting from a cliff into the underworld, a dead body speaking prophecies, smoke drifting through a deserted valley at dawn. Even the credits are brilliant, as Celtic symbols and splodges of psychedelic colour fade in and out to the mad accompaniment of Donovan’s “Hurdy Gurdy Man”. Everyone involved with this brainmelt of a show seems to be having the time of their lives, and the scope of the subject matter – the Roman invasion, after all, took decades to complete – suggests that it could run and run. I, at least, could watch this lot for years. Honestly, there’s nothing else like it.