The Federation's starship Enterprise is tasked with escorting Medusan ambassador Kollos and psychologist Miranda Jones to a meeting with a Medusan vessel. Medusans are non-humanoid beings whose appearance is so horrifying that it can drive humanoids insane and eventually kill them. Kollos travels in a carrier to protect the crew, and First Officer Spock uses a unique visor to safely interact with him, while Jones relies on Vulcan-style mental discipline.

During a dinner with Captain Kirk and senior officers, telepathic Jones shares her objective of forming a mind link with Kollos to utilize his unique senses and navigational skills. However, she soon senses someone with murderous intent nearby. Jones later discovers her associate, Lawrence Marvick, is the potential killer. He attempts to murder Kollos, but the sight of the Medusan drives him insane. Marvick then seizes control of the Enterprise's engines, causing the ship to speed beyond Warp Factor 9.5 and enter a mysterious void. Afterward, Marvick dies while accusing Jones.

With no navigational guidance, the crew is unable to return home. Kirk suggests using Kollos's navigational expertise, and Spock offers to mind-meld with him. Miranda Jones opposes the idea, but when it's revealed she is blind, Spock proceeds with the plan. Unfortunately, Spock accidentally views Kollos without the visor after successfully returning the Enterprise to known space and goes mad. He is sedated and taken to Sickbay, where his condition worsens.

Jones tries to help Spock mentally but initially fails. Kirk accuses her of not genuinely wanting to help due to jealousy, which angers her. Motivated by this, she tries again and successfully restores Spock's mind. Eventually, the Enterprise reaches its destination, and Jones and Kollos prepare to leave. Jones thanks Kirk for his insight and acknowledges the joy of the mind link. Kirk presents Jones with a rose, reminding her that every rose has thorns as they part ways.