I like movies that are inventive, take chances, and show interesting sociological ideas. This movie ticked all of those boxes. Sadly, I don't like movies that have internal continuity problems, and this movie ticked that box, too. It could have been a really amazing movie, but there were some issues that kept me from loving it. But I had to give it extra credit for the effort. If you like weird-idea sci-fi movies with explosions and action, like Total Recall, then I think you'll enjoy this one. Just don't think TOO hard about it.
What Happened to Monday is features future Earth facing problems we're just starting to think about now. Overpopulation forces us to embrace GMO crops in order to feed the masses, but there's a backfire: it causes pregnancies with lots of twins. Faced with a growing crisis, the government steps in with a solution: only one child per family with the others frozen to awaken in the future once we've solved all of our issues. When a grandfather loses his daughter while giving birth to identical septuplets, he decides to hide them by raising them as the same person, each taking a day. He names them after the days of the week with a single public persona that each embodies on their day. They do this successfully for 30 years, but Monday suddenly goes missing, causing their world to come tumbling down as they figure out what happened to Monday.
So while I loved the concept and many of the consequences, I had too many issues with the details to allow me to really love the movie. I could have loved it, but the execution was just off for me, so I just liked it. Just as an example, 30 years in this environment, and little changed in society. That's hard for me to believe. I love Noomi Rapace, but I think she was miscast for this. It should have been 20 years or so (and she should have had an accent different from the rest of the people around her). These are nitpicky things, but they stopped me from giving it a 6 or maybe even 7. It was just too hard to ignore them for me. It reminded me of the Justin Timberlake movie, In Time. Cool concept, but I just couldn't believe the society and choices being made. I liked this one more than that one because the issues weren't nearly as bad as for In Time (you can easily steal the life from anyone sleeping and no one has arm protectors? Silliness.) So I would say give Monday a try, but don't get your expectations up.