Spock risks the Enterprise when he remains in unstable space claimed by the Tholians in order to retrieve the Captain who appears to have phased to another universe.
I hate to call this one episode "painful to watch" but it is an example of season 3 tending toward more formula and less solid content.
The premise is exciting, but its too clogged with dramatic flourishes and odd characterizations. Several things are very poorly-explained. Its good to explore the idea of how Spock and McCoy would be at each other's throats in the absence of Kirk, but much of the dialog is all over the map. What are they really arguing about? Spock's decision to wait for Kirk to phase again? He has negotiated the original amount of time with the Tholians, and merely reacts to the Tholians firing on the Enterprise seconds later. McCoy's accusations would have more weight if they made more sense, he had not said a thing about objecting to Spock waiting for Kirk to re-appear previously. The phasing between universes is interesting, and the pressure of hostile aliens makes the situation even more tense. However, the script here calls for yet another threat, an ill-defined "space-sickness" (that somehow has caused every last crew member of the Defiant to kill one another) but is solved on the Enterprise by a Theragen derivative that merely suppresses impulses to the brain like alcohol. So much for a unique effect of inter-dimensional phasing, its an easy and poorly explained disease that McCoy says is "communicable" like a virus. And when the Enterprise faces its critical moment, a quick power use and its "thrown clear" of the Tholian web. What? Lazy and not well-written.
Two nice moments are Kirk's final instructions to Spock and McCoy, and the finale has the usual touch of comedy between the three friends. Oh, and Triox compound is mentioned in a moment of continuity with "Amok Time". Not nearly enough to save this one.