This story picks up at the end of Where No Man Has Gone Before. Myfanwy has (secretly) received just a touch of the psionic enhancement that corrupted Gary Mitchell and Elizabeth Dehner, and Dorothy is being tutored in the Vulcan language by Spock. Dorothy additionally has a strong mental shield; says Spock: "I have noticed that you are the only person aboard this vessel who can approach without my noticing. If a door opens behind me and no one comes in, it is liable to be Conway."
Later, Dorothy has gone down on an away mission (in case her linguistic skills were needed), and when she returns, she claims not to be able to see the other two members of the away team who have just beamed up. She talks in Vulcan to Spock, to convince him something funny is going on, so he collects a few security officers, determines that the other two away team members are psionic illusions, and takes them all down to the planet in a shuttlecraft (bringing Myfanwy along just for kicks, I guess).
They all march along following Dorothy's directions, since she's the only one who can see what's really there. They find the two missing crew members, Spock has an imaginary sword fight with Tharks straight out of Burroughs, and they all join arms and return to the shuttlecraft.
The plot's pretty thin, and there's more than a hint of Mary Sue between Dorothy and Myfanwy, but it's serviceable.