Some years ago human colonists from Battrul fought a war with the wolflike inhabitants of Canvine, and since then a neutral zone has been established between their territories. Vega Station is a hotel and casino on the edge of the neutral zone, established as a private Battrulian enterprise and tolerated by the Canvines. The Doctor visited Vega some time ago, and when the TARDIS returns him to the station, he and Fitz make a bet to determine who will win the most in a single week. Fitz promptly loses his entire stake in a single spin of the roulette wheel, and spends the week sitting at the bar wondering how he’s going to explain himself to the Doctor. Still uncomfortable around alien species, Fitz manages to insult “Bigdog” Caruso, a Canvine tour guide.
After years under a military dictatorship, the people of Battrul have voted the liberal President Robyn Drexler into office, but there are still some on her staff who also served under the previous incumbents. Her chief of staff, General Browning Phillips, particularly believes that she will see the Canvines in a new light after her visit to Vega Station to attend an exhibition of paintings by the late Toulour Martinique. Meanwhile, the secret executives of Vega meet to discuss special security arrangements for the President’s visit -- arrangements which can be used to deal with another secondary problem which has just cropped up after lying dormant for several years.
While playing in the casino, the Doctor and his companions meet some of the people who live and work on Vega. Vermilion Kenyan, a professional greeter, works to convince customers of the casino to spend more money; her own ambition is to earn enough to get off the station as fast as she can. She introduces them to the fussy casino manager Harris Stabilo and noted antiquarians and card cheats Newark Rappare and Ambrose Forster. Meanwhile, the noted assassin Hazard Solarin arrives incognito on Vega and smuggles customised glass weapons through security without being detected. His employer contacts him to offer him a secondary job, and they agree to meet in the casino; but due to a series of misunderstandings, Fitz is mistaken for Solarin and ends up hired to kill someone he’s never met...
Tullus Gath, a former protege of Martinique, has been placed in charge of the exhibition by Henri Blanc, Martinique’s former sponsor and the owner of most of the paintings. Rappare and Forster contact her, claiming to have found a previously undiscovered Martinique in their collection -- a self-portrait. Gath, deeply disturbed by the news, contacts Blanc, who agrees to come to Vega in person to verify the portrait’s authenticity. Rappare and Forster, satisfied, return to their shop to set the rest of their plan in motion. Meanwhile, Stabilo is attacked in his office and knocked out, catching only a glimpse of a giant, hairy beast. Bigdog denies any knowledge of the attack, but it seems that the attacker could only have been a Canvine.
Vermilion is supplying drugs to the station’s engineer Macleb when another such beast attacks them, kills Macleb and knocks out the station’s power. Sam, alone in the Martinique exhibition when the lights go out, hears the sound of movement and then a tearing sound; when the lights are restored, she becomes convinced that one of the paintings now has two demons lurking in the background where it originally had one. The Doctor, meanwhile, convinces Cage to let him help investigate, but no DNA traces are found at the scene of the attack, and Vermilion claims not to have recognized her attacker. Cage suspects that Vermilion is protecting her friend Bigdog, but is forced to release her back to her quarters while she recovers from the shock of the attack. Vermilion takes the opportunity to speak with Gath, and when Cage next tries to find and question her further, she can find no trace of Vermilion anywhere on the station.
Sam takes Fitz to the Martinique exhibition, where they’re both disturbed by the famous painting “Murdering art” -- not for its depiction of a man’s brutal murder by two demons, but by the screaming woman in the background who looks very much like Vermilion Kenyan. They intend to ask the Doctor for his opinion, but as they depart, they are shot at by Solarin. Although they manage to escape, Solarin has determined to his satisfaction that they are not rival assassins. Now he must find out who they are, and how their presence affects his primary mission. Meanwhile, further minor acts of sabotage occur on the station, and in each case a shaggy, hairy figure is glimpsed shambling away; when the Doctor investigates, he finds smudges of oil paint at the scenes. Despite the appearance of danger posed by the Canvines, President Drexler is still determined to visit the station.
Sam and Fitz take the Doctor to see “Murdering art” for himself, only to find that it has been stolen. Gath, horrified, explains that the painting is Martinique’s last; it was found in his studio after Martinique’s own brutal murder, and it is believed that the unknown killers found it and used it as inspiration for their own crime. The painting has been stolen by Rappare and Forster, who need to determine its age for their own plan to succeed, but their scanning systems are confused; although the painting dates roughly to the time of Martinique’s murder, one particular screaming woman in the background appears to have been added only hours ago. Unnerved by the arrival of a mysterious man who seems to know that the antiques in their store are all forgeries, Rappare and Forster rush their work and leave the painting in an alcove where it will be discovered and returned to the exhibition.
A surveillance post is attacked and the two operators on duty killed, and the Doctor finds smudges of oil paint at the scene. Cage claims that this is an internal surveillance post, but the Doctor wonders why, if this is the case, the surveillance tapes have failed to provide any clear evidence about the crimes. Meanwhile, Fitz tests the tracking unit he was given by the assassin’s client, and finds it leading him straight to the Doctor. He is forced to admit the truth, and the Doctor confirms that the photograph of Fitz’s target is that of his fourth incarnation. Fitz decides to remain out of circulation in the Doctor’s hotel room for the moment. Meanwhile, President Drexler arrives on Vega, and slips away from her bodyguard for a secret meeting with Bigdog -- while someone else from her retinue meets with Gath and Blanc to ensure that everything is prepared.
The Doctor returns to the exhibition to look for Vermilion in the painting, but the figure is facing away from the window. Martinqiue’s techniques were aided by digital scanners which overlaid images of the subjects upon the canvas, to which Martinique later added his own interpretations. The Doctor asks to see the scanners Martinique used, and Gath and Blanc offer to demonstrate by generating a picture of the Doctor. The scan generates a perfect, photo-realistic image of the Doctor, but Gath and Blanc seem taken aback for some reason. The Doctor departs, feeling that the scan was somehow akin to the process of digitilisation used in matter transmission. Gath and Blanc, unsure why the process failed to work on the Doctor, awaken the Devourer, a demon from one of the paintings, and send it to kill him. It finds Fitz instead, and when Fitz slashes at the demon with a knife from a room service tray the demon’s skin peels away to reveal it is just paint over canvas. Fitz manages to trap the Devourer in a lift and flee, but is then confronted by Bigdog, who demands to know where Vermilion is. Fitz tells him she’s trapped in a painting, and Bigdog decides he’s an idiot and lets him go. But the stranger who earlier unnerved Rappare and Forster contacts Bigdog with bad news. Meanwhile, Sam visits the exhibition to look for more clues, but is caught alone by Gath and Blanc...
Fitz describes Solarin to the Doctor, who decides to contact him, pretend to be the original client and call off the hunt. Solarin isn’t fooled, but agrees to meet the Doctor at the Martinique exhibition, where he tries to kill him. Fitz, meanwhile, realizes who it was who hired him and rushes to the exhibition to warn the Doctor. The Doctor uses the uncannily realistic portrait of himself to catch Solarin off guard, but Stabilo then arrives and holds them at gunpoint. Fitz explains that it was Stabilo, in disguise, who hired him. Stabilo drops his fussy demeanour and admits that he is the secret CEO of Vega Station, which is in truth a spy satellite set up closer to the Canvine border than the treaty allows. Over the years its purpose has become an open secret, as has Bigdog’s status as a Canvine spy, and the peace is kept by allowing Bigdog to conduct counter-espionage while both sides pretend not to know what the other is up to. Solarin has been hired not to assassinate the President but to protect her, as Stabilo and Cage don’t believe that Phillips has her best interests at heart.
Bigdog arrives and spots Vermilion in “Murdering art”, just as the stranger had told him -- and the Doctor and Fitz discover that Sam has been added to another painting. While Fitz and Bigdog remain in the exhibition trying to talk Vermilion and Sam out of the paintings, the Doctor goes to the casino and bets the credit chip he kept from his last visit to Vega against Rappare and Forster’s Martinique self-portrait. When Rappare and Forster discover just how much is coded to the chip they fall over themselves to acquire it, but lose the game and are forced to hand over the portrait. Since they had been cheating, they conclude that the Doctor must have cheated as well, and thus logically they are morally clear to steal the portrait back. But when they break into the Doctor’s hotel room they find him waiting, along with the mysterious stranger -- Toulour Martinique himself.
Sam is able to emerge from the painting, but it’s too late for Vermilion, who has set into the background and can never escape. Martinique explains that his process is akin to matter transportation, but that it transforms the subject into paint and canvas. The subject is either transported into the painting directly, or becomes permanently linked to the copy. Gath and Blanc have been releasing the demons in order to cause havoc which will be linked back to the Canvines; Vermilion must have recognized the demon which attacked her and attempted to blackmail them. Their ultimate motive is still a mystery, and the Doctor decides to force the truth from them by threatening to destroy the paintings. Rappare reluctantly reveals that he wove an incendiary strip into the backing of “Murdering art” before it was returned; he and Forster intended to set the exhibition aflame to inflate the value of their own forged Martinique.
The Presidential reception begins, and the Doctor, Sam and Fitz attend it along with Rappare and Forster. But security has been tightened up, and a guard confiscates the incendiary remote trigger from Forster. The Doctor manages to smuggle in paint stripper in a hip flask, but Drexler arrives before he has the chance to speak with Gath and Blanc. Gath and Blanc then lock everyone into the exhibition and release the demons. In the confusion which follows, Solarin dies saving Fitz’s life and Forster is also killed. Stabilo informs the Doctor that his voice can override the locks, and as the others flee, Stabilo reveals the Doctor owns Vega station; since he could potentially have bankrupted them if he cashed in his credit chip from his last visit, they had no choice but to sign the ownership over to him in absentia. Over the years, Stabilo and his associates have decided that they quite like running the hotel and casino, which is why they decided to have the owner of the credit chip killed when he returned.
Phillips is revealed to be the mastermind behind the plan, having learned of the possibilities of Martinique’s technology while he was government liaison to Blanc’s research council. He forces the others back into the exhibition, where the Doctor bargains for his life with the credit chip; since it is keyed to his biology and the digitising process doesn’t seem to work on him, Phillips agrees. But the Doctor also requests the self-portrait of Martinique; unaware that it’s a forgery, Blanc refuses to let it go and torches it rather than let the Doctor have it. The monsters, who regard Martinique as their creator, are furious, and then Martinique himself arrives and reveals that he is in fact a copy of himself which he painted before his own murder -- at the hands of Gath and Blanc, who killed him to inflate the price of his paintings. “Murdering art” was painted after his own death, and the Doctor uses his paint stripper to reveal the faces of Gath and Blanc under the demonic faces of the killers -- and that the true title of the work is “Murdering Martinique”. The monsters turn on Gath and Blanc in a rage, and in the resulting confusion, Fitz finds the incendiary remote in the pocket of a dead guard, and activates it. “Murdering Martinique” bursts into flames, as do Gath and Blanc, and as the sprinklers go on, a demon grabs Phillips and drags him back into its painting, where he will remain forever.
Martinique retreats to peaceful solitude within a quiet painting, while the Doctor decides to leave the credit chip trapped in Phillips’ hands. Drexler reveals that she really came to Vega to sign a permanent peace accord with Bigdog, the Canvines’ representative. Rappare decides to sell his store, and Fitz suggests that he find a new career making genuine copies instead of illegal forgeries. In order to atone for his earlier insults of, Fitz buys Bigdog a drink. Honour is satisfied.