Kev and Rob are possessed by two entirely different manifestations of pure evil and have a knock-down, drag-out fistfight in sickbay over the slightly confusing but gratifyingly adventurous events of "Through the Lens of Time" (SNW). Having regained their senses, they investigate other instances of body possession in "Return to Tomorrow" (TOS), "Power Play" (TNG), and "Tears of the Prophets" (DS9).
SNW 3×05 Through the Lens of Time
SNW 3×06 Those Old Scientists
Roger Korby
TOS 1×09 What Are Little Girls Made Of?
SNW 3×01 Hegemony, Part II
Body possession
TOS 2×20 Return to Tomorrow
TOS 1×24 Space Seed
Diana Muldaur (as Ann Mulhall)
TOS 3×07 Is There in Truth No Beauty?
TNG 3×26 The Best of Both Worlds
TNG 5×15 Power Play
DS9 6×26 Tears of the Prophets
TOS 1×19 Arena