Space beacons on the space lanes are being blown up and plundered for precious argonite by a gang of space pirates led by Caven, and his associate Dervish. The Earth Space Corps cruiser V-41 notices the destruction of the beacon and, with General Hermack and Major Warne in charge, sets out to apprehend the pirates. Another beacon is destroyed despite their best intentions, and the fragments are stolen using rocket propulsion. Hermack deploys troops to all nearby Beacons to prevent another robbery.
The TARDIS crew has arrived on Beacon Alpha Four shortly before the pirates reach it. Caven and his men mop off the security force on the Beacon, and the pirates seal the time travellers in part of the Beacon before blowing it to pieces. Fortunately the beacon falls into discrete, sealed pieces and the Second Doctor, Jamie and Zoe find themselves inside one. The eccentric Milo Clancey, in his aged ship the LIZ-79, rescues them – but they cannot retrieve the TARDIS, which is in a separate segment taken by the pirates.
The nearest inhabited world is Ta, dominated by the Issigri Mining Corporation, whose leader is Madeleine Issigri. The firm was founded by her father and Clancey, and the latter is now suspected of Dom Issigri’s murder, though nothing has been proved. Hermack visits Ta, believing that Clancey, whom he suspects of being the pirate leader, will end up there in due course – and he is right. However, Hermack leaves just as Clancey and the TARDIS crew reach Ta. Zoe has plotted the trajectory of the segments of Beacon and believes they were destined for Ta too, and as per usual the Doctor and his companions soon find the pirate headquarters. They evade capture and make contact once more with Clancey.
Meanwhile Caven forces Dervish to reroute some of the beacon fragments to Lobos, a frontier world where Clancey has his base, so as to throw suspicion on the prospector. It is clear someone has tipped him off about the Corps suspicion of Milo Clancey. Hermack and his crew see through this ruse, but it takes time, and they spend hours orbiting Lobos while the real action is taking place on Ta.
When the Doctor and his party reach Madeleine Issigri’s offices it becomes clear she is in league with Caven, and the Doctor and his friends are once more imprisoned. Their prison is the study of Dom Issigri – alive but frail and scared – and it takes time for him to recover his wits. Madeleine has meanwhile decided to break her alliance with Caven, and does so by radioing Hermack to bring his troops to Ta. Caven reasserts his authority by telling Madeleine her father is alive and threatening to kill him unless she returns to her compliant self. She responds by contacting Hermack again and telling him not to come to Ta.
The Doctor and his friends have meanwhile escaped, taking the weak Dom Issigri with them, and head to the LIZ-79. Caven has thought ahead and forced Dervish to cut the oxygen supply to the ship. As only Milo and Dom board the ship, theirs are the lives in danger, and Caven’s callousness finally convinces Madeleine to support him no longer. The Doctor, Jamie and Zoe save their friends and Dom Issigri makes contact with Hermack, persuading him of the truth of the situation.
Caven now gets desperate, threatening to destroy Ta, the Issigri base and the orbiting V-ship by means of a series of strategically placed bombs. The Doctor manages to disengage the triggering device in the nick of time, while Major Warne blows Caven and Dervish’s ship to pieces. As Hermack’s ship lands, Madeleine looks forward to a reunion with her father, but knows she will also be imprisoned for her part in the conspiracy; while the Doctor and his companions prepare to seek out the TARDIS on one of the fragments of the Beacon.