'Psirens' is the first episode of series 6 and features one of Red Dwarf's periodic reconceptualisations of the show - at the beginning of 'Psirens', we find out that Red Dwarf itself has been stolen (or possibly Lister just can't remember where he parked it) and the crew have therefore lost Holly, and are on a mission to recover both it and her(/him). They are following the ship's vapour trail, which requires them to fly through an asteroid belt.
In the belt, they find evidence of crashed ships and a message scrawled with the victim's own intestines ('someone who badly needed a pen') - PSIRENS. The following conversation ensues:
LISTER: This entire belt is swarming with some kind of genetically engineered life form who can alter your perception, telepathically. They're called Psirens. Like with Ulysses in that ancient Turkish legend.
KRYTEN: I believe the legend was Greek, sir.
LISTER: Whatever. Some country big on curly shoes and hummus. The point is, they use this power of illusion to lure you on to the asteroids, strip the ship of anything they can use and suck out your brains.
RIMMER: They shouldn't bother us, then. There's barely a snack on board.
KRYTEN: We can't turn back. We'll lose Red Dwarf.
LISTER: Look, we'll be through the belt in three, maybe four hours. We've just got to be on our toes. They'll try and tempt us, scare us, break our morale - anything to force us down on to the rocks. Just be alert.
The script I pulled from the internet said 'hummus' but I always thought I heard Lister say 'eunuchs'...
Odysseus (Ulysses in Latin) listening to the Sirens while tied to the mast of his ship: the other crew have stopped up their ears (mosaic from Bardo Museum, Tunis)
Obviously this is based on the Sirens, one of the best known Greek myths. In mythology, it was the Sirens' song that lured sailors onto the rocks (their appearance is less important, though they are often depicted with wings). These Psirens (the silent 'P' tells you it's Greek, like 'psychology'!) are a little more sophisticated - they can appear in many different forms in order to get you to come near enough for them to suck out your brains. So, for the Cat, they appear as a group of women who have lost their men and need seed-spreaders (a set-up which also has a basis in Greek mythology), for Lister, they appear as Kochanski, who has used his DNA to have twins called Jim and Bexley (after Jim Bexley-Speed; I have to confess, I would never have recognised Clare Grogan as Kochanski if Lister hadn't said so, she looks so different here!), and for Kryten, one appears as his creator, whose every order he must obey.
Although the Psirens appear in various forms (usually beautiful women) their true form is as large, insectoid, beeping monsters, resulting in a delightful scene where Lister, feeling desperate after being deprived of sex for more than three million years, snogs one. (Apparently Craig Charles had complained that it wasn't fair that Rimmer got all the action in series 5, since his character was dead, and this was the writers' response).
Somewhat grainy image of a Psiren from the crashed ship's black box recording
Lister and the Cat (the only crewmembers who have edible brains) are eventually saved by Kryten - the Psiren thought she had got rid of him by having him crush himself in the waste disposal unit, but all it does is make him cube shaped.
Red Dwarf used aliens which could appear as anything as plots several times over the course of the series (most successfully in 'Polymorph', but 'Polymorph II: Emohawk' was pretty funny too). In this case, the focus on the sexual lure of the Psirens gives the episode a slightly different focus and keeps it fresh (plus the fact that there are several of them). The Psirens don't work only through sex - in one very funny scene, one of them poses as Lister and reveals itself by actually playing the guitar well - but it seems to be their favourite method of attack. The Sirens of classical mythology aren't particularly sexy (unless wings turn you on) but they have become associated with sexyness over the years, as their irresistable call has been interpreted as a sexual lure. If I had more time, I'd look into exactly how that happened - I suspect it involves Romantic poets. Anyway, this is pretty good episode - perhaps it suffers in comparison to the same series' 'Gunmen of the Apocalypse', but it's funny and it introduces us to the show's new format, with added danger and tension as a result of the need to chase Red Dwarf, very effectively.