The Cetacean finds an underwater forcefield, which shelters a race of air-breathers with a strange clicking language. Mr. Schubert forces the Clicks to mine crystals for him, but taking the crystals will cause the field to collapse and kill them all unless Mark can intervene.

An enzyme from a rock fossil Mark finds on the sea floor with the properties of changing people's personalities gets into C.W's coffee while Mark and Elizabeth are sent to recover a lost survey probe with a deadly self destruct booby trap.