A space beacon is destroyed and this registers on the instruments of the Interplanetary Space Corps Cruiser, V41. Commander General Nicolai Hermack (Jack May) believes the beacon has been stolen by a band of criminals because it is composed of the valuable mineral argonite. He decides to abandon the ship's present mission to investigate the thefts and protect the remaining seventeen beacons. 

The pirates are already setting charges on another beacon. Dervish (Brian Peck) is worried that they are bound to be caught if they continue to raid the beacons so frequently. The pirate's leader Caven (Dudley Foster) tells Dervish that the Space Corps are far too occupied to catch them and they will never have a better chance to get rich. 

The V41 makes contact with four nearby beacons. Hermack decides to put the ship in orbit around the planet Ta - the base of operations for the Issigri Mining Company - where it can keep a close eye on the beacons and still be able to use the facilities on the planet for rest and recreation. An unrecognised ship is spotted near beacon Alpha Seven and the V-ship goes after it. The pirate ship speeds off and the beacon breaks up in front of the V41. Each segment of the beacon starts to follow the pirate ship due to small rockets being fixed to each section. 

Hermack is furious. He decides to put men on each of the remaining beacons even though the living conditions will be cramped and uncomfortable. Four men led by Lieutenant Sorba (Nik Zaran) will man beacon Alpha Four. Major Warne (Donald Gee) orders them to shoot anyone boarding the beacon without authorisation. 

The TARDIS materialises inside of one of the beacons. Although they haven't reached their planned destination the Doctor is fascinated by the structure and begins to explore. Believing the beacon to be unmanned the Doctor is shocked to find himself and his friends under attack from Sorba and his men. The gunfire prevents the Space Corps men hearing the arrival of the pirates at one of the airlocks. Sorba's men are ambushed by the pirates but manage to signal the V-ship which changes course back to the beacon. Sorba is taken prisoner and the pirates prepare to blow up the beacon. 

The Doctor, Zoe and Jamie have taken shelter in a sealed area of the beacon. When the Doctor thinks it is safe to emerge they try the door but find that it won't open. 

The pirates leave before the V-ship can arrive and the beacon is broken up. 

Hermack is frustrated that at such large distances from their base, the V-ship cannot maintain its boosters for long enough to catch the pirate ships. Another unregistered ship enters the area which the V-ship computers identify as a decrepit C-class freighter. The ship is piloted by an old-style frontier space miner called Milo Clancey (Gordon Gostelow). Clancey tells Warne that his ship's registration is Liz-79. Hermack believes Clancey could be one of the pirates and orders him to be brought on board the V41. 

The Doctor, Jamie and Zoe discover that they are being moved to an unknown destination and that the TARDIS is inside another fragment of the beacon. 

Clancey introduces himself as the head of the Milo Clancey Space Mining Company and demands to know what the Space Corps is doing about the Argonite pirates. He has sent a number of reports in about the pirates and knows that they are in possession of a Beta dart, one of the fastest ships in the Galaxy. 

Hermack allows Clancey to go on his way and later tells Warne that he believes that the old man is the ring leader of the pirates and intends to follow his movements carefully. A minnow ship is launched - piloted by Warne - Hermack orders him to follow Clancey's ship without being discovered. 

The Doctor is concerned by the falling levels of air in their section of the beacon and plans to pull the sections of the beacon back together using magnetism. Unfortunately, the magnets repel each other and they end up shooting off in the wrong direction. 

Hermack visits Madeleine Issigri (Lisa Daniely) on the planet Ta. Issigri has managed to make the worked-out mines on the planet into the most productive in the Galaxy. She promises to aid the General in his hunt for the pirates. Hermack tells her of his suspicions regarding Clancey but she is dismissive even though some people had claimed that Clancey had been responsible for the death of her father. 

Warne reports that Clancey is docking with one of the pieces of the beacon. Hermack decides that this is the evidence he needed and orders Clancey's arrest. The Doctor, Jamie and Zoe are nearly out of oxygen when Clancey cuts his way into their section. He holds them a gunpoint believing them to be pirates. He soon realises that he is mistaken and hurries them onto his ship as Warne begins to fire warning shots. 

Clancey releases copper needles into space which jams the minnow's scanners and the Liz-79 escapes. Hermack orders the V-41 to launch all the remaining minnows to hunt for Clancey. 

Clancey goes to Ta where he has a long forgotten personal dock where he can repair the ship. Zoe uses her mathematics skills to work out the location of the pirate base and deduces that it must also be on Ta. Despite Clancey's warning about the dangerous tunnels the friends head off to find the pirates. They stumble on the base and are chased into a tunnel where they fall into a deep pit. When they recover they find they are not alone, with them is the badly injured Lieutenant Sorba. Sorba tells the travellers that there is no escape but the Doctor notices a fragile water bowl which could not have been thrown into the pit without breaking. 

Warne docks with the V-41 and explains to Hermack how he was fooled by Clancey. Hermack orders the ship to travel to Clancey's base on the planet Lobos. 

The pirates find that one piece of the beacon is missing. Remembering that Sorba mentioned a group of strangers, Caven decides to interrogate the man further. Searching around the pit walls the Doctor discovers a lock operated by sound. Using a tuning fork the Doctor tries to find the right note and eventually is successful. Clancey enters the pit through the door, he tells the Doctor he has been trying to rescue them. They all leave taking Sorba with them. 

The V-ship comes across the pieces of the Alpha Four beacon heading towards Lobos. They also spot a Beta Dart ship nearby, piloted by Dervish. Dervish contacts Caven and tells him he has been spotted but will attempt to lose the V-ship in the camouflage zone. 

The pirates discover that the Doctor and his friends have escaped and chase them through the tunnels. Clancey leads them towards the offices of the Issigri Mining Company. He tells the Doctor that he had believed that Madeleine was behind the raids but that he recognised the pirate leader as the notorious criminal Caven. The Doctor sets a booby trap for the pirates and then joins the others in Madeleine's office. Clancey is attempting to warn Madeleine about the pirates when Caven and his men burst into her office. Sorba tries to reach a gun and is shot. 

Madeleine reveals that she is a part of the pirate organisation but is horrified by murder of Sorba. She stops Caven from killing the others and they are taken away to be imprisoned. 

Warne has reached Lobos and discovered that Clancey's run-down mining operation is not big enough to house a Beta dart ship. He deduces that they have been sent on a false trail away from the pirate base. Hermack calculates that the beacon sections were originally on course for Ta. 

The Doctor, Clancey, Jamie and Zoe are pushed into a dark room. In the room they find a dishevelled man that Clancey recognises as his old partner Dom Issigri (Esmond Knight). Issigri explains that he has been held prisoner since Caven and his cut-throats took over his operation. 

They plan an escape and manage to make their way back to the Liz-79. Clancey and Issigri reach the rocket first, but Caven has had a remote control unit fitted to the ship and it takes off before the Doctor, Jamie and Zoe can get aboard. Madeleine is shocked to discover her father is still alive and pleads with Caven to spare him. Once the Liz-79 is in space, Caven cuts the oxygen supply and the two old men collapse. Back on Ta, Caven goes in search of the Doctor leaving Dervish to guard Madeleine. The Doctor, Zoe and Jamie overpower Dervish but not before he destroys the remote control unit. The Doctor succeeds in repairing the unit before the Liz-79's oxygen finally runs out. 

Issigri and Clancey are unharmed by their ordeal. The V-ship closes in and orders them to cut their engines. The two men tell Hermack the truth about the pirates and the General sends a rescue force to land on Ta. 

Caven traps Madeleine and the others by locking the main door to the mining company's offices. He tells her that he has ordered the setting of bombs in the planet's atomic fuel dump and that they will soon die in a massive explosion. 

The V-41 is too large to land in the forty minutes that remain before the explosion so Clancey heads back to Ta once he rips out the remote control device. Hermack sends Warne out in a minnow after Caven's fleeing Beta Dart ship. Clancey lands on Ta and releases the Doctor and the others. The Doctor goes to the fuel dump and manages to defuse the bomb with seconds to spare. Warne simultaneously attacks the Beta Dart and the ship explodes. 

The repentant Madeleine is sent back to Earth for trial and Clancey gives the Doctor, Jamie and Zoe a lift back to the TARDIS which is orbiting Lobos in one of the segments of the Alpha Four beacon.

_{Synopsis from Doctor Who: The Second Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}