3 or 4 stories involving couples at different stages of their lives. Interspersed.

As it continues, you see that this guy is cheating on this girl, but in this couple he cheated on her for a while, but then got back together and are fine now, etc.

Then, finally, it starts getting really obvious that they share similarities, it's demonstrated how one couple literally becomes the next.

So, like, this couple might go through some crisis where they switch back and forth between the two sets of actors for the two couples before finally settling on the second one.

The next, like, get back together after being seperated some after cheating. After talking through that, they're the next couple.

The next couple is looking at a house, and when they decide to get it, they can be seen in mirrors and stuff as the next couple. When they finally move in they have a moment where one of them is the last, and one of them is the next. The next one says "Oh my god, we're home owners now" or something, and the other one hugs them, having now begun the next phase.

If I do children this would be the next phase.

Etc.