The TARDIS arrives close to an unstable area on the interstellar cruise ship “Empress”, which has emerged from hyperspace at the same co-ordinates as the trade ship “Hecate”. This has caused a dimensional crossover that the Doctor and Romana realise must be repaired and he offers his services to detach the two craft. Rigg, captain of the “Empress”, is suspicious of the Doctor’s alias as a representative of Galactic Salvage, but nevertheless agrees to let him try and separate the two craft by reversing the smaller craft at full thrust. The Doctor is accompanied on this task by Rigg’s co-pilot, Secker, who, it becomes apparent, is a drug addict. He is hooked on the organic substance Vraxoin, whose origins are unknown, but whose properties are lethal and dangerous. Secker heads off alone into the unstable area and while there is attacked by a clawed monster and left for dead. K-9 arrives from the TARDIS and is tasked with cutting through the locked ships.
Also aboard the “Empress” are a zoologist named Tryst and his assistant Della, with their CET (Continual Event Transmuter) Machine, which actually stores portions of planets on electro-magnetic crystals. Their collection is large and also ethically dubious. Their most recent stop was on the planet Eden where one of their expedition was killed, but both Tryst and Della are reticent to provide too many details. Romana, however, examines the Eden projection when she is on her own and is sure she has seen eyes staring out at her from the dark and forbidding jungle. When she later looks at the projection again an insect appears from within it and stings her.
The Doctor and Rigg find the wounded Secker and send him to the sickbay where he dies. When the Doctor finds Secker’s drugs stash he is prevented from acting when someone stuns him and steals the evidence. Once he has recovered he returns with Rigg and K-9 to cut through the power source. Once a hole is made a roaring creature appears, flexing its vicious claws.
K-9 repels the creature with blaster fire while the Doctor and Rigg refit the segment of the craft. The Doctor continues to try and separate the two ships while also trying to source the Vraxoin on the craft. Rigg is positive there are no drugs on his craft, but events soon take a sinister turn, which proves him wrong. When Romana wakes up an unseen hand spikes her refresher drink with the drug, but it is Rigg who ends up drinking it. He soon starts to show signs of addiction and altered perception and heads off alone as his cravings grow.
After the Doctor and K-9 fail once more to separate the two ships, he spots a silver-suited stranger and pursues him through the passenger deck and into the blurred area between ships. The Doctor loses his quarry, but manages to relieve him of a radiation band which he dropped and proves that he was on Tryst’s expeditionary team in the past. The clawed monsters are loose near there. When the Doctor flees back to the “Empress” he discovers Rigg has become addicted and it becomes apparent that Tryst thinks Della is the smuggler, in league with her late partner Stott, who was killed on Eden. Two Azurian Customs and Excise officers now board the craft, Fisk and Costa, and start to suspect the Doctor of smuggling because of the traces of Vraxoin in his pocket. The Doctor and Romana make a break for it and head to the CET Machine room where they evade capture by leaping directly into the projection.
Inside the projection, the Doctor and Romana are menaced by the jungle plants and must hide to avoid the clawed monsters, which obviously originate from Eden and roam freely in this section of the planet. They soon meet up with the fugitive previously sighted by them both, Stott, who takes them to his sheltered cubicle. It seems that he is a Major in the Intelligence Section of the Space Corp and has been hiding in the projection for the past 183 days while he tries to establish the source of the Vraxoin, which he knows is from Eden but not from which organic source. He also names the vicious creatures as Mandrels. The trio exit the projection and return to find the “Empress” under siege from the marauding beasts, which have now started killing the passengers (as shown in the picture above). Rigg too is killed, shot down by Fisk during a mad search for Vrax.
The Doctor, Romana and K-9 evade the creatures while trying once more to separate the two spacecraft. In the process, the Doctor incinerates one of the Mandrels, which disintegrates into raw Vraxoin. The beasts are evidently the source of the drug. He reapplies himself to the technical task and, with the help of his companions, the ships are finally parted – but the Doctor disappears from the “Empress” in the process.
The separation has been a success, with the elusive Dymond having returned to his own craft at the right time. Fisk warns him not to leave too quickly, but Dymond is keen to get away. The Doctor is also on the “Hecate”, having been caught up in the separation of the two ships, and, without being noticed, soon finds evidence of Dymond's complicity in the drug running project. Dymond returns to the “Empress” by shuttle, and the Doctor smuggles himself on board. Back on the “Empress”, Romana finds Della and confides in her that Stott is still alive, but Della is soon arrested by the Customs men and they are separated.
The Doctor rejoins Romana on the “Empress” and says he has seen evidence that the smugglers are planning to use an intuca laser to transport the Eden projection between the two crafts. He is now certain that Dymond’s ally is Tryst and, when Stott arrives, he also confirms the source of the Vraxoin. Fisk and Costa turn up to arrest the Doctor, but Stott pulls rank and warns them to back off. In another part of the craft, Tryst is reunited with Della and confesses all about his part in the smuggling racket. She flees when a Mandrel arrives and distracts Tryst, who is rapidly trying to escape with Dymond. They head back to the “Hecate”.
The Doctor has meanwhile rounded up the Mandrels using K-9’s dog whistle, having worked out they are pacified by ultrasonics. He leads them all back into the projection and then slips out, leaving the creatures trapped. His next task is to reverse the CET transfer process to stop the smugglers getting away with the Vraxoin supply. After allowing Tryst and Dymond to transport the Eden projection to the "Hecate", he activates the CET and traps them within a new projection – they are ready for the Customs Officers to walk in and arrest them. With the ships separated and the drug runners caught, the Doctor and friends slip away back to the TARDIS.