When Hercules and the boys are asked to escort a consignment of treasure across the desert to Helios, the intrepid trio have some serious reservations. As Hercules puts it: “The desert is home to murderers, thieves and cut-throats. It would be an act of lunacy.” However, everybody has their price, and with the prospect of a good pay day, they are soon carrying their cargo across the arid wasteland in a caravan with several other travellers, unexpectedly accompanied by Pythagoras’s brother Arcas, who Pythagoras is strangely unhappy to see. He later admits to Jason that they do not always see eye to eye.
The treasure is a wedding gift from the father of a young man, Philemon, who is pledged to marry a woman from Helios, and who makes the journey with the friends, along with a musclebound mute called Otus, who wears the sign of a murderer. But as they travel, Hercules is convinced something is not right, and suspects it could be the presence of a young woman, Baucis, in the caravan.
After the travellers are attacked by brigands, Baucis saves Philemon’s life, and they all take refuge in a cave that is sacred to the Furies, the vengeful spirits of those who have been murdered.
Unbeknownst to the others, Arcas summons up the Furies to take revenge on the man who murdered his and Pythagoras’s father. Meanwhile, Baucis attempts to steal the treasure, only to be thwarted by Jason’s quick-thinking.
The travellers are besieged by the Furies. Arcas believes this means that Otus must have murdered his father. However, Pythagoras admits he was responsible, killing their father accidentally while trying to prevent him from attacking their mother. Arcas is shocked to learn what happened, and will not believe it was an accident.
As the Furies attack again, Hercules is badly injured, and the travellers fear for their lives. Jason asks Arcas to call off the Furies, but he refuses, and Pythagoras insists the others leave him behind to suffer his fate. Jason stands by his friend, however, and eventually Arcas forgives Pythagoras and asks the Furies to cease their vengeance.
The boys are safe, but as they reach Helios with the jewels, Philemon informs them there will be no wedding. He has fallen in love with Baucis. Once again, the three friends are left without any money, but remain in good spirits as they return to Atlantis.