Murderous Fury (The Tholian Web)

[Bridge]
COMPUTER: Computer working.
SPOCK: Compute the next period of spatial interphase.
COMPUTER: Next interphase two hours twelve minutes.
SCOTT: Two hours and twelve minutes? That power leak has unbalanced the ship and she's starting to drift. I can't hold her in place that long.
SPOCK: We must try, Mister Scott. The fabric of space is very weak here. If we disturb it, there will be no chance of retrieving Captain Kirk alive.
CHEKOV: I don't understand what's so special about this region of space.
SPOCK: Well, picture it this way, Mister Chekov. We exist in a universe which co-exists with a multitude of others in the same physical space. At certain brief periods of time, an area of their space overlaps an area of ours. That is a time of interphase, during which we can connect with the Defiant's universe.
UHURA: Mister Spock? We will be able to retrieve the Captain at that time, won't we?
SPOCK: Yes. However, the dimensional structure of each universe is totally dissimilar. Any use of power disturbs it. If we are not extremely careful, we shall lose the Captain and become trapped ourselves.
UHURA: I see.
CHEKOV: And die like him?
(Chekov leaps up, pushes Spock away and, screaming, attacks Sulu. Scott grabs him and McCoy arrives to help. Spock grabs his face.)
SPOCK: Chekov! Chekov!
(Spock neck-pinches him into silence.)
SPOCK: Security guards to the Bridge.
UHURA: Aye, sir.
MCCOY: What happened, Mister Spock?
SPOCK: I was attempting to explain to Mister Chekov the nature of our problems in this situation when he attacked.
MCCOY: Did anyone notice anything peculiar in his behavior before this?
SULU: I think he had several spasms of pain.
MCCOY: Did he seem to be getting hysterical?
UHURA: Doctor, he seemed more angry than frightened.
SCOTT: He had no cause to be angry.
(Security arrive.)
MCCOY: Take Mister Chekov to Sickbay. Put him under restraint.
(Scott and the security guards leave with Chekov.)
SPOCK: Lieutenant Uhura is correct. There was murderous fury in Chekov.