This interlock is located marking the frontier of Brittany and Normandy, in the north of France. Officially it's in Normandy.
The history backs until 4th century and the church is devoted to archangel St. Michael. There is few people living there, a lot of shops and the church. While a church is, usually in plain terrain, as the space, there is very limited, they had to construct the church upwards... so you can find different parts of a church, in different floors. You must consider that the size of the island is 4 km2...
When I was there, I slept in a house in Saint Malou (a town with a nice history of piracy near Mont Saint Michel), and the owner told us a nice trick to avoid lot of people and visit it fast. Once you enter the main door of the town, you must turn right and pass another great door, once passed that door, search, at your left, the first small street and take it, it's so small that one person fits... two don't. Go up that street, and you'll pass between house, at a side of and old cementer and you'll reach the church in a direct way (just in front of a defibrillator...). Then, you can visit the church and then, when exiting, you can follow the great street and take a look at shops or stop at a bar... but you must avoid this when going up, is what everybody does and you loose a lot of time. I followed this and I found that it was very useful!
Also, beware of the tides... they go up fast and they usually advice the time you must take out your car... or you'll have to fish it. ;). Somebody told me that every week, at least one or two cars must be taken to repair because of the owner left it and the tide came...
Being at that place, with that amount of history, how it remains and get adapted to time and how people made the effort of building that beautiful place is something that makes you reconsider your situation and what are you doing.
A great place to visit.