The basic idea here is that, once every 300 years, the modrons march an army through all of the other planes for no apparent reason. Most get killed off but a few make it back to Mechanus; no one knows why they do it. This time, though, they're marching off schedule for no apparent reason, and behaving oddly to boot.
This is far from being an exceptional adventure pack -- in fact, it's close to unusable. The adventures involved are all tied together and occur in sequence, but with wide gaps of time between them; nevertheless, each one is consecutive in its level requirements. This is basically your campaign if you run it. The book's no good as a sourcebook; this Modron March is so weird that even what little general info you get isn't applicable unless you use the full background.
If you want a coherent campaign with more creativity and better writing, get Tales from the Infinite Staircase. If you want a combat-centered campaign that shakes the multiverse but actually makes sense, get Hellbound: The Blood War.<br><br><b>LIKED</b>: The second scenario is completely playable and a lot of fun, although it makes the modrons into out-and-out villains. This is a problem, because most of the remaining scenarios are about helping them. It also actually gives (good-aligned) PCs reasons to participate in the story.<br><br><b>DISLIKED</b>: In the rest of the scenarios are often blatantly coercive or just assume the PCs will accept missions with little motivation; there is no troubleshooting advice if they don't, just "if they don't take the job, there's no adventure." The scenarios repeatedly contradict each other about the basic way the March works and the conditions involved. Two scenarios are just nonstop combat; there are NPCs to talk to, but nothing to accomplish by doing so. Another scenario requires the PCs to seek vengeance for the death of an amazingly annoying NPC. The writing is poor and unusually gory, even for Planescape; only Hellbound was more bloody-minded, and it made SENSE there.
What's more, this is a metaplot adventure, which permanently changes the universe -- if you want to keep using later Planescape adventures, you have to either run this or assume that it happened in the background. In addition, this adventure never even tells the DUNGEON MASTER the reason for the premature March; you're expected to buy the next module in the series to find out!<br><br><b>QUALITY</b>: Poor<br><br><b>VALUE</b>: Disappointed<br>