When Tuvok and Chakotay return from a brief shuttlecraft journey, the crew discovers that an alien presence may have returned with them as strange events begin threatening the ship. A new low for the series, "Cathexis" again stresses how much Voyager, unlike its DS9 counterpart, lacks story arc development along the series as a whole.
Beginning with an utterly pointless and unmotivated teaser in which Janeway takes some recreational time in the holodeck, this installment continues to offer scenes that offer virtually nothing in terms of character development. "Cathexis" does not choose one character to focus on, but throws them all together into a ridiculous plot that gives none of them enough to do.
When the shuttlecraft returns, both Chakotay and Tuvok have suffered injuries. Tuvok recovers quickly, but, alas, Chakotay is brain dead because his neural energy has been mysteriously "drained." Tuvok explains that they encountered an alien vessel which attacked them and then retreated into a nebula.
Janeway orders a return to the nebula to investigate. But en route, Paris apparently makes an unauthorized course correction to avoid it. The mysterious part is that he has no memory of ever doing so. A similar occurrence happens upon Torres in engineering when she shuts down the warp core but believes she did nothing of the sort. When the Doctor examines them in sickbay, he discovers brain wave patterns that suggest they were under an alien influence when carrying out these disputed actions. Apparently, the alien can occupy anybody's mind and control their actions. No one can be trusted.
From this point, "Cathexis" turns into a series of disjointed events with poorly executed plot handling. The third act manages to work in elements of what seem to be the beginnings of a paranoid thriller, but the idea never gets off the ground outside of the one scene which introduces it. Instead, we get some standard revelations and a number of weak contrivances, such as the gratuitous crashing of the main computer and the ejection of the warp core.
In order to prevent the ship from being seized via an alien takeover of her own mind, Janeway transfers the command codes to the Doctor, since the computer presumably cannot be affected by the alien's influence. But someone deactivates the Doctor's program and renders the plan useless. Janeway decides to divide the command codes and give half to Tuvok, but a bizarre scene in which the alien begins seizing the minds of any bridge officer it encounters and then specifically attacking Tuvok hints that he may be the key to part of the mystery. This scene is interestingly photographed, as the alien begins body jumping from one person to the next. Unfortunately, the way it ends—Tuvok stunning everyone on the bridge with his phaser set on wide beam—has an inappropriately comical effect.
As evidence mounts against Tuvok, suggesting that he lied about the shuttle incident, it becomes clear that he is directly under an alien's influence. The other body-jumping alien turns out not to be an alien at all, but Chakotay's missing neural energy "somehow displaced," as Janeway puts it. This allows Chakotay to take control of other people's minds (as a countermeasure to the Tuvok-alien) in an attempt to save the Voyager from nebula-inhabiting, neural energy-thieving alien baddies. The idea might have sounded good in a writer staff meeting, but is completely ridiculous on screen. Chakotay using Neelix to rearrange the stones on a medicine wheel to make a "map" that helps the crew escape the nebula is even more ridiculous (and really strains credulity).
An atypically weak direction by Kim Friedman doesn't help either, as this episode fails to produce the slightest amount of excitement or urgency at every turn. If Voyager wants to do mundane sci-fi concepts like alien body snatchers, it had better find a better angle to take than this one does.