In the latest take on this classic franchise, the lead slasher has been locked up for quite a long time when it starts. This film takes place forty years after the original timeline wise. It starts with basically an intense scene of the terrifying monster locked up in Smith's Grove Sanitarium. There are spoilers for the film after this point, you are warned.
It's only after an intense mask prompting that the character gets into action. This was caused by two British reporting Podcasters, invited by the good doctor of our lead monster. Now, here's basically the best guess in Halloween (2018), how Michael Myers escaped captivity. The general release happened during a fairly standard move via bus to a new facility. Time changes, sometimes you need to upgrade. They should have really had better protection for this type of crazed man, like really. If you look closely at the clues and generally the attitude, it would seem that his psychiatrist Dr. Ranbir Sartain, a former student of Dr. Samuel Loomis set this all up.
The character reveals his full nature later on, but the clues are there. Right from the start he's quite happy to get Michael going with the return of that mask. He had it brought in, and made sure it was pushed to his patient. From there, it would seem that he sabotages the bus transfer. He insisted on sitting with him on the way, and most likely assisted in the escape. It explains him being the only survivor.
He had an odd fascination with the care and protection of getting this killer into his natural habitat of Haddonfield. That's basically the most concrete reason as to why he was able to break out and go crazy. It was just the release advantage Michael Myers needed to eliminate any extra threats on his way back to the hometown. Anyways, I hope this helps explain things somewhat. You can read our review of the film below and check out the movie hubs area for neat genres such as our horror coverage.
Read our Halloween (2018) Review
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