Ostensibly a follow-up to last week's 'The Marriage of True Minds', this week's Atlantis in fact couldn't be more different from its predecessor. While that was another outing for the show's tried-and-tested 'quest' format - though a strong example of the form - this is a rather spectacular indulgence in outright horror.
Having (just barely) survived an arrow to the chest, the witch Pasiphae (Sarah Parish) casts black magic, while our heroes hunt for Jason (Jack Donnelly), who - along with Medea (Amy Manson) - took a fall into the unknown.
But instead, they're greeted by a demon Dion (Vincent Regan) - Ariadne's adviser resurrected by way of necromancy. If the title of Howard Overman's script, 'The Day of the Dead', put you in mind of George A Romero's 1985 horror flick, then you're thinking along the right lines - here be zombies!
Atlantis doesn't shy away from the grim trappings of the genre either. These zombies are hungry for human flesh and if they catch up with you, death is a mercy - the only way to prevent you turning into one of them.
It's no idle threat - of course, none of our regulars meet their end, but two sympathetic supporting characters fall prey to the end, and our heroes also face the moral quandary of being forced to kill their injured friends before they transform.
This is still going out pre-watershed on a Saturday night, so there's nothing in 'The Day of the Dead' as spectacularly gruesome as you might see on The Walking Dead - decapitation, the classic means of dealing with a zombie, is swapped out for a less gruesome strike to the heart.
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But this is certainly as violent as Atlantis has ever been and there's some genuine frights on offer - particularly Dion sinking his teeth into Diagoras (John Light) and that character's subsequent transmogrification - that means this is probably unsuitable for very young children.
For the rest of us though, it's a ghoulish treat, with the undead hordes a formidable threat brought to 'life' by way of some excellent make-up effects - a real physical menace always trumping a CGI creation, for my money.
With the undead swarming, Jason forms an unlikely but inevitable bond with Medea, allowing 'The Day of the Dead' to serve up some more moral complexities and a properly surprising cliffhanger climax along with its scares.
It benefits the central characters to paint them in shades of grey, so both the good guys' intolerance of Medea and her eventual betrayal - mortally wounding Ariadne (Aiysha Hart) - come as welcome developments.
Pasiphae's fate is still left frustratingly unclear by the time the credits roll, but that's one of very few flaws in an excellent episode of Atlantis - I'm a sucker for zombie movies, so was predisposed to enjoy this, but I'd feel confident in labelling this an all-time series high.
Certainly, anyone who's dismissed this show as weak, insubstantial fare 'for children' should give 'The Day of the Dead' a look.
