Colonels Dalton and Chaudhry visit Southend on UNIT business; Chaudhry enjoys the fresh sea air, but Dalton just wants to get back to London. They are here to meet archaeologist Barney Hendrick, who has just unearthed evidence of human sacrifice in a Saxon burial ground. Chaudhry left a message with one of his students; Hendrick has lost several mobile phones while on digs in Eastern Europe, and doesn’t use them any more. Delighted to have visitors from the UN, Hendrick invites them back to his caravan for a coffee while his students work on the dig, using marker sticks to determine how deep the skeletons are in the earth, and thus how old. Hendrick flirts shamelessly with Chaudhry while explaining that he’s had to camp out here to protect the dig from local idiots ever since a reporter did a story on “buried treasure.” Dalton is revolted to learn that the Saxons here apparently practiced human sacrifice, but Hendrick points out that Dalton’s own name is of Saxon origin. His ancestors would have emigrated from North Germany about 1400 years ago and settled in a valley up north; thus, “dal” for dale and “ton” for village.

Irritated by Hendrick’s attitude and by his flirting with Chaudhry, Dalton corrects his misapprehension: they’re not from UNESCO, but UNIT, and this visit isn’t about funding the dig, but about the damaged mobile phone that turned up on the beach near the dig. The phone’s late owner washed up on Ministry of Defence land at Foulness, and he’s not the only one; there have been a few unexplained deaths in the vicinity, and the evidence suggests that the victims died out on the mud flats. Hendrick concludes that they were “trickers,” cockle-pickers, but he has little interest in the investigation. After showing Chaudhry and Dalton where the phone was found, he takes his leave of them and rushes into town for an appearance on a local radio programme, The Place To Be. He turns on the charm for Lesley, the female interviewer, and laughs off a racist e-mail from a listener claiming that the buried Saxon warlord is just another bloody immigrant here to take away jobs.

Dalton and Chaudhry retire to a Chinese restaurant called the Lucky Dragon to discuss their findings. Dalton can’t believe that cockles are such a big deal, but according to Chaudhry, they’re worth millions to the seafood industry -- and collecting them requires hours of painstaking labour, a job often foisted off on illegal immigrants. On the surface, it appears that some cockle-pickers got caught out on the mud flats at high tide... but that doesn’t explain why the bodies were partially eaten. Dalton would prefer to believe that this is the work of a human serial killer or perhaps an escaped animal from a zoo, but Chaudhry, who has more experience with UNIT, suggests that the archaeological dig may have woken something ancient and paranormal, perhaps a grendel from the poem Beowulf.

Their Albanian waitress, Anni, has mixed up their orders, and while Dalton investigates, Chaudhry calls Lt Hoffman back at UNIT HQ. Hoffman reports that Palmer from forensics is still working on the damaged phone; in the meantime, Hoffman has found an interesting 999 call recorded by Essex police. He sends the sound file to Chaudhry’s phone, and then goes through the other calls he’s received. Chaudhry tells him to send the crackpots to ICIS, but takes an interest in a call from Francis Currie asking for a comment on human traffic through Tilbury docks. Chaudhry cuts lunch short and informs Dalton that the “snake heads,” Chinese gangs who smuggle illegal immigrants into the country, were caught trying to smuggle a single Kosovan in through Tilbury. And the 999 call sent by Hoffman appears to have been made by a Kosovan, a panic-stricken man who is repeating the word “vrykolak.”

While Dalton visits Tilbury, Chaudhry returns to Hendrick’s dig; she knows he’s been in Eastern Europe before, and she needs him to translate the 999 call. Hendrick flirts with Chaudhry again upon her return, offering her a drink of Scotch to celebrate his divorce; he met his wife on a dig in Eastern Europe, and she’ll become his ex at the end of business hours in Bulgaria. Chaudhry politely turns him down and asks him to listen to the 999 call, which he dismisses as a joke of some kind; the man, apparently terrified, is reciting the Lord’s Prayer to ward off a vrykolak, a form of Albanian vampire. According to legend, the vrykolak breaks its victim’s bones to suck out the marrow, and only visible to other vrykolaks and to dhampirs, the half-breed offspring of a vrykolak mating with a human woman. Hendrick believes that the number of self-styled dhampirs is going to increase, and not because vrykolaks are said to have a great interest in mating with human women. Many Yugoslavian single mothers sought asylum in Britain in the 1990s, and they’d probably prefer to tell their children that their fathers are vrykolaks rather than admit that they were raped in prison camps. There are dhampirs in Albania -- but they’re usually con men, snake oil salesmen who visit superstitious communities out in the backwaters of the country where people still believe in curses and evil spirits. Whenever an illness or misfortune strikes the village, the dhampir turns up, blames the misfortune on an invisible vrykolak, and pretends to slay it -- for a price. Hendrick claims that it’s all nonsense, but he is taken aback when Chaudhry shows him photographs of the dead Kosovans, whose injuries match his description of vrykolak victims...

At Tilbury, Dalton questions the Kosovan charged with illegally entering the country, one Goran Dhampir. Goran is wary of the newcomer, but Dalton convinces him that it’s in his best interests to co-operate. This is currently an international matter and thus under UNIT’s jurisdiction, but since Goran is currently on British soil, it could be considered a matter of national security -- which means that Goran would end up dealing with ICIS. Goran agrees to co-operate, and Dalton questions why he tried to enter the country illegally; apparently he tried to enter legitimately four weeks ago, but was turned back at the border for lying about his criminal record. Goran denies that he is a criminal, and insists that the Albanian courts that charged him with fraud are too closed-minded to accept that the service he provides as a vrykolak hunter is legitimate. Dalton scoffs, which is no more than Goran expected, but Goran insists that he is the only one who can see the vrykolaks -- creatures with glowing eyes, teeth like needles and a head like a snake. However, Dalton is more interested in discussing the person who sponsored Goran’s legitimate attempt to enter the country: Kevin Lee, a Southend restaurateur who’s suspected of connections to the illegal cockle-picking industry.

Chaudhry follows another lead to Kevin’s house, where he reluctantly lets her in when she claims to be from the UN. He’s surprised to see that a woman named Chaudhry is white, and she’s surprised to find that a man with such a broad English accent and name is of Chinese ancestry. This embarrassing moment aside, Kevin shows Chaudhry around his apartment, quickly running through all the evidence that his marriage to Anni is genuine and that she’s not a “passport vampire” using him to get into the country. However, he clams up when he learns that she’s not there to investigate his marriage; she’s here because his name came up in connection to the deaths of the cockle-pickers. The 999 call was made from Kevin’s mobile phone, which he’d reported stolen, and UNIT forensics has determined that Kevin’s home number was the last one called from the mobile that washed up on the beach. Kevin refuses to answer Chaudhry’s questions, and tries to kick her out of his flat, insisting that this is a local matter -- but Dalton then drives up with Goran, who has given in and agreed to co-operate fully.

Kevin is unwilling to co-operate at first, but Chaudhry and Dalton convince him that they really have no interest in his marriage, his connections to the snake heads, or any taxable income he may or may not have reported. They just want to solve the mysterious deaths, and if they can’t do that tonight, the case will be turned over to ICIS, and Kevin and Goran will be caught up in the cogs of a hostile bureaucracy obsessed with matters of national security. Kevin gives in and admits that he’s been hiring illegal immigrants to pick cockles; first the Chinese, and then the Kosovans whom he’d met through his wife. However, the Kosovans got greedy, set up their own gang, and somehow brought a vrykolak into the country to get rid of their competition. Once the vrykolak had finished off the Chinese, however, it turned on the Kosovans who’d brought it here. Kevin has seen the results, which is why he brought in Goran; nobody will care about a few missing immigrants, but it will be summer soon, the tourists will arrive in Southend, and it will be much better for all concerned if the vrykolak has been dealt with by then.

To Chaudhry’s surprise, Dalton is willing to concede that this could be a genuine vrykolak. He’s done some research, and has learned that the last reported sighting of a vrykolak was in 1959; the political situation in Kosovo since then means it’s unlikely that any other incidents would have been made public. Chaudhry thus contacts Hoffman to report that they’re going vampire hunting, but turns down his offer to send backup; ICIS is already nosing around, and she and Dalton want to get this over with quickly. Dalton checks the tide tables; high tide isn’t until 2:00 am, so Goran and Dalton will be able to stake out the mud flats for most of the evening. Chaudhry will keep watch from the dig, and Kevin insists upon coming along, as he blames himself for starting all of this.

Thus, as night falls, Dalton finds himself standing out on the mud flats with a self-styled vampire hunter. He is armed with night-vision goggles, but Goran does not believe that the vrykolak will show up on them, pointing out that there’s plenty of surplus military hardware in Kosovo. He also warns Dalton that bullets will only wound the creature, not kill it. Chaudhry, Hendrick and Kevin are at the dig, and Dalton becomes irritated when he realises that they’re clustered around a heating element, eating hot food from the Lucky Dragon. Back at the dig, Hendrick continues to flirt with Chaudhry, but dismisses the “vampire hunt” as nonsense. He doesn’t even believe in the Silurians, convinced that they’re an invention by the media; given time, the government will claim that the Silurians have turned on them, and will use this as an excuse to invade some foreign country. Hendrick believes that there’s no need to invent monsters, aliens and conspiracies to explain the evil humans do to themselves.

The debate is interrupted by the sound of gunfire out on the mud flats, as Goran has spotted the vrykolak and is trying to direct Dalton’s gunfire. Dalton sees nothing through his night-vision goggles, and is helpless to intervene as Goran drops to the ground, thrashing about as he struggles with the invisible creature. Chaudhry tries to monitor the situation via her own night-vision binoculars and Dalton’s mobile phone, and Hendrick sits back, vastly entertained and convinced that Goran is having them all on. Goran uses his stake to wound the creature, and as he chases it across the mud flats, Kevin tells Dalton to aim above the footprints; the creature itself is invisible, but Dalton should still be able to see the marks it’s leaving in the mud. But Dalton can see no footprints... which means that there isn’t really a vrykolak out here after all. And when Goran returns, wiping sweat from his brow, he claims that the vrykolak disintegrated to dust when staked, leaving no evidence behind.

Back at the dig, Hendrick laughs at the UNIT officers’ foolishness. Chaudhry calls Dalton, telling him to calm down before the police show up, but the humiliated Dalton lashes out at her, telling her to get the civilians off the beach. Chaudhry, irritated, hangs up on Dalton, and he realises too late that though he can see a figure on the beach through his night-vision goggles, she can’t. The irritated Chaudhry doesn’t answer when he calls her back, and he rushes towards the dig, shouting out a warning and firing warning shots. At the dig, Kevin returns with coffee and tells the others that he’s going down to the beach to answer a call of nature -- and Chaudhry realises that the figure Dalton saw wasn’t Kevin.

Hendrick has had enough of this, and he insists that Dalton is just pulling their legs and that there’s no such thing as a vrykolak -- but even as he speaks, the invisible creature lunges out of the shadows and takes him down. Dalton orders Kevin to cut the power to the caravan, and the lights go out, leaving them all on equal footing in the dark -- except for Dalton, who identifies the vrykolak through his night-vision goggles and shoots it. The injured vrykolak pursues Emily into the trenches, saying something in Kosovan about family -- but, unable to understand it, she stakes it through the heart using one of the students’ marker sticks. Its body indeed disintegrates to dust, leaving nothing behind.