When returning from a conference, Picard, Data, Geordi, and Troi attempt to rendezvous with the Enterprise, but before they do, they discover strange things happening in the space-time continuum. Troi experiences everyone else in the room freezing in time for a few seconds. Later, Picard reaches for a bowl of fruit that has suddenly gone rotten, and screams in pain as his hand suddenly ages several weeks in a few seconds. (In a nice touch, his fingernails have grown to an alarming length.) Time in this region is moving at different rates within variously sized pockets of space. When their Runabout finally reaches the rendezvous point, the Enterprise is frozen in time, apparently in the middle of a battle with a Romulan warbird.
"Timescape" takes the idea of time travel to its logical next step (note that I did not say "logical conclusion," as there is likely always room to go further) by having space-time shattered into multiple levels of backward, forward, accelerated, and decelerated. Geordi engineers a way to surround the bodies of the Runabout crew with a technobabble field so they can visit the Enterprise without becoming frozen in time. This allows them to walk around the decks of the frozen Enterprise (in scenes reminiscent of TOS's "Wink of an Eye") so they can try to figure out the mystery of what went wrong. Frozen Bad Things Happening include Crusher being phasered, the bridge apparently under siege by a Romulan boarding party (but things are not as they appear, as we learn), and a warp core breach in progress in engineering.
Like a lot of other conceptual sci-fi examples of what I like to call Good Brannon Braga (see also "Cause and Effect" and "Frame of Mind") the truth of "Timescape" is all in the details. Like the most entertaining of TNG tech adventures, this story knows that to keep us engaged it can't shy away from some fairly involved details that clearly explain what's going on. But at the same time, it has to walk a fine line so it doesn't drown in pure exposition. Data is always perfect for this task, as in the scene where he explains the Enterprise is not actually frozen in time, but moving forward extremely slowly. Apart from tech details, some zany humor sure doesn't hurt. Qualifying as a classic moment in my book is when Picard, experiencing a moment of "temporal psychosis," draws a smiley face in the gas cloud of the warp core explosion, and then laughs maniacally.
The story is honestly more fun as an unsolved puzzle than it is once all the reasons for the shattered space-time are made clear. (For the record, an alien race from another time continuum mistook the Romulan engine core for a black hole, which they attempted to use to incubate their young, which had a disastrous shattering effect on space-time after the Enterprise attempted to initiate a power transfer into the Romulans' engine. Feel free to go back and read that sentence again; I'll wait.)
The story's momentum flags somewhat in the last couple acts once all these answers come at us. And, of course, the complexities of shattered space-time ultimately become very easily manipulated, as, I suppose, they must. (You haven't seen anything until you've seen a tricorder essentially become a rewind button.) But then, this storyline also means that we get to see the Enterprise explode, and then unexplode when time rewinds. What more could you ask of the guy who blew up the ship four times in "Cause and Effect"? "Timescape" is a fun and well executed sci-fi yarn of space-time zaniness done in the TNG tradition of procedural investigation. It doesn't mean much of anything, but, hey, that's perfectly okay.