This film starts with a great premise. Sadly, it doesn't come anywhere near fulfilling its potential due to some extremely shoddy writing.
My main complaint is that the characters react to their discovery in ways that make them unrelatable and unlikable. More importantly, these people don't feel authentic. Not only would no one I know behave like them, I have a hard time imagining that anyone would. There are some halfhearted efforts to make us feel sympathy towards them, but those are superficial, not to mention coming too late in the narrative to have the intended effect. The relationships feel forced. There's scene about blowing up a pile of money that's one of the dumbest things I've seen. (Seriously, who has dreamt of blowing up a million dollars since they were six??)
OK, so the characters are bad, but what about the science fiction element? Frankly, the writer didn't think that through, either. Alt worlds vary most in "creative" pursuits like music and art, we learn. Yet somehow that has little to no effect on architecture, city planning, or any other aspect that might keep worlds from looking and feeling identical. How many humans are children of artists or musicians? Entire family trees might exist or not because of art. And if you have different people in the world, you guessed it: everything else is probably different, too.
Later, without explanation, we also learn that some worlds DO vary in technology... but those worlds are superficially similar, too. We're told the tech is "world-changing" -- and it would be -- yet it comes from a world where the same old house and its attic still exist. That's just poorly thought out.
Never mind that these people are somehow able to walk into a parallel world and get a 5G connection on their phone.
As for the action, there is little to speak of. It includes people rolling around with a gun, which of course discharges. Amateur hour, there. Conflicts devolve into tedious physical scuffles and gun-pointing. In a movie about alternate dimensions, we deserve much better.
The ending is tacked on without any attempt at suitable explanation, almost like a focus group thought the main plot's conclusion was anticlimactic, which it was.
In short, this movie's a valiant effort, but it seems to have been written by someone who didn't nearly have the chops to pull off this high concept successfully or give us characters who weren't paper thin. I can't recommend Parallel even to people like me who really like movies about alternate dimensions.