The Vulcan name for the blood fever, the final stage of pon farr in which reason gives way entirely to the drive to mate or fight.
Plak tow is the last and most dangerous phase of the pon farr, the mating cycle that overtakes adult Vulcans every seven years. A Vulcan in the plak tow can barely speak, is aware of little except the rival or mate in front of him, and will die if the fever is not resolved.
The term is introduced in the Original Series episode Amok Time. On Vulcan, T'Pau tells Kirk and McCoy that Spock is deep in the plak tow and can no longer be reasoned with, which is why he cannot refuse when T'Pring chooses Kirk as her champion. The shock of believing he has killed his captain breaks the fever.
The condition returns in the Voyager episode Blood Fever, where Ensign Vorik passes it to B'Elanna Torres through a telepathic bond, and it is resolved by ritual combat between them.