The 'new' Zep, Jason Brown and stunning new scientific adviser Vana, are sent to investigate the murder of the President of Aquaberta - a vast underwater city.
The people on Planet X are dying. They've run out water. The old president knows the whereabouts of secret supplies but he is assassinated just as he's about to reveal the location.
Zep is called up to find why the Tree of Yath is dying. Without the fruit from this one tree the people will starve. Is it natural causes or is it sabotage?