The Enterprise is ordered to pursue a group of anti-establishment idealists who have stolen a space cruiser and made off for the mythical planet Eden. When the group pushes their stolen ship beyond its limits, the Enterprise is forced to rescue them by transporting them aboard. This merry band of space-hippies includes an insane leader (Dr. Sevrin), an academy drop-out and former love interest of Chekov (Irina), and the son of a Catullan ambassador (Tongo Rad). With the Federation undergoing fragile treaty negotiations with the Catullans, Kirk is ordered by Starfleet to treat the dissidents with “extreme tolerance.” Kirk finds the group and its leader too difficult to deal with while Spock maintains a deep curiosity about their ideals. Kirk appoints Spock as liaison for the group during their stay on the Enterprise. Dr. Sevrin demands to be taken to Eden, but Kirk refuses on the grounds that his orders from Starfleet dictate that the group be taken to the nearest star base. While investigating this strange group of free spirits, Dr. McCoy and Mr. Spock discover that not only is Dr. Sevrin insane, he is also the carrier for a potentially lethal disease. His desire to abandon technology and join the “primitive” inhabitants of planet Eden could well result in their destruction. As a precaution, Dr. Sevrin is quarantined, but Spock agrees to plot a course to the mythical planet of Eden in exchange for his cooperation. Meanwhile, Chekov inadvertently gives away some vital information about auxiliary control to his former classmate, Irina, and she and the rest of the followers soon free Sevrin and hijack the Enterprise. The group takes the Enterprise to Eden (which, to complicate matters, happens to be in Romulan space), establishes orbit and steals a shuttle craft to land on the surface while the Enterprise crew is incapacitated and left to die. But the planet, while as beautiful as any of them imagined, also has a deadly side.