The Enterprise crew receives a distress call near the site of the loss of the USS Essex, which went missing two centuries before. Surely the crew is long dead, but Riker and a team take a shuttle down to the moon's surface to investigate whether there may be any life. When they find themselves stranded with an approaching energy storm, O'Brien does some fancy transporter work to rescue them, but not before some mysterious energy enters the away team's bodies.
Data, Troi, O'Brien, and Riker return to the ship, but it quickly becomes clear that their bodies have been taken control by hostile entities — except for Riker, who for some reason has not been possessed. The other three go to Ten Forward and take hostages, ordering Picard to take the ship closer to ... oh, who am I kidding? I don't care about the details of their demands, and I've already forgotten what they were.
"Power Play" is a watchable and competent but by-the-numbers hostage situation as filtered through various sci-fi/fantasy conceits. It's also an example of the tried-and-true Trekkian standby that allows the regular characters to act outside their normal personalities because of those sci-fi/fantasy conceits. Troi is the leader of the hostage-takers. (Insert Troi-bashing joke here, such as, gee, she makes a more credible leader here while possessed by an alien influence than she did as herself in "Disaster.")
There's an alleged notion here that the body-possessors are actually the souls of the Essex crew, trapped in some sort of purgatory. Picard & Co. mostly scoff at that notion, but the story seems to want to play out that string anyway. (The body possessors actually turn out to be disembodied prisoners in a disembodied-alien penal colony.) But mostly, the episode is content to simply deal with this as a straightforward hostage situation, documenting the crew's progress and setbacks in their attempts to negotiate with and/or thwart the hostage-takers. Your mileage may vary, but for me it was hard to be either excited or much disappointed by something that mostly achieved what little it set out to do.