X-Men: Days of Future Past

In a future where mutants have been hunted almost to extinction, the survivors send one man's mind back to 1973 to stop it from ever starting.

Two casts, two decades, one film. The older actors play out an ending in a future that has already lost, and the younger ones play the week in 1973 where it could still be prevented, with Logan as the only person who exists in both.

It is the most structurally ambitious entry in this continuity and also the most pragmatic. Time travel gives the series a way to keep what worked and set aside what did not, and the film uses it on its own history in front of you.