I just want to make sure I understand. These movies all have the exact same plot, right? Ethan Hunt is accused to be a double agent and has to go underground to find out who the real traitor is.
I've seen at least 4 but can't differentiate between any of them and barely remember the last one at all.
The way you can distinguish them is through the big setpieces mostly, where they take place and their directors styles.
The first one has the Langley heist, with Cruise having to access a computer on a wire and he can't touch the ground or drop anything. Also there's a big TGV setpiece. Jean Reno, Emmanuelle Béart and Jon Voight are involved. Takes place in Prague and London.
II is the John Woo one, with Thandie Newton and more importantly doves, a lot of them. It opens with Cruise climbing in MOnuments Valley and when he gets on top, a helicopter comes in and some guy literally shoots his mission at him (in the form of self-destructing glasses). THen it's mostly in AUstralia, and it's very John Woo.
III is the J.J. Abrams one, with PSH as a villain and all of a sudden Ethan Hunt has a wife (Michelle Monaghan) who doesn't know about his real job and gets abducted, because of course. The big setpiece is on a bridge (with rockets being thrown at Hunt & Co., among other things), and the film also involves Keri Russell, Laurence Fishburne and Simon Pegg, who sticks around for the next ones. The ending is in Shanghai, with Hunt jumping from a tower to a next, because of course.
Ghost Protocol is the one with the Burj Khalifa sequence. Also Léa Seydoux, and some silly stuff in India at the end.
Rogue Nation is the one with the Vienna opera sequence and the one where he has to stay underwater for an eternity. Rebecca Ferguson is there, and most of it is in London and gets back to the more spy-roots of the first film, though it's still all about the setpieces of course.