Director Bryan Singer returns to the franchise he launched with 2000's X-Men and the sequel X2 — and with a $200 million-plus budget, making Days of Future Past the priciest and most complicated X-Men film to date. It's not hard to see where the money went: Past, in which a distant-future Wolverine (Hugh Jackman) is sent back to the '70s to prevent war, combines the casts of both the first X-Men trilogy (Jackman, Patrick Stewart, Ian McKellen, Halle Berry, etc.) and 2011's prequel X-Men: First Class (James McAvoy, Michael Fassbender, Jennifer Lawrence) in a time-travel story involving two time periods, six countries, and hundreds of giant killer robots. ''We have to deliver, and that's really hard,'' says Lauren Shuler Donner, who's produced all of the X-Men films. ''Plus, we don't use guns, we use powers. The power is a visual effect. So by its very nature it's going to be pricey.''

Past aims to boost X-Men's fortunes by bringing back marquee stars such as Jackman and Berry as well as younger ones like Lawrence and Fassbender whose profiles have risen in recent years. ''The hope is that Days of Future Past will broaden the audience for X-Men such that it will motivate potential spin-offs even more,'' Kinberg says.

In the X-Men fan community, the comic story line ''Days of Future Past,'' written in 1980 by Chris Claremont and John Byrne, is hallowed ground. It imagines the time-traveling brain of Kitty Pryde and a desolate future in which X-Men are hunted by machines called Sentinels. When captured, the mutants are either killed or placed in internment camps. It seemed like the perfect vehicle for a film that could link Stewart and McKellen with the First Class cast.

There was just one seemingly insurmountable problem: In the X-Men movieverse, present-day Kitty is played by 27-year-old Ellen Page. So in the movie version of Past, it's gruff, unaging Wolverine who returns to his 1973 body to stop Mystique (Lawrence) from assassinating the inventor of the Sentinels, Bolivar Trask (Game of Thrones' Peter Dinklage). Keeping Trask alive prevents a devastating war between mutants and humans — and keeps Mystique among the do-gooding mutants. As Kinberg explains, ''A lot of people have an emotional investment in her not going to the dark side.'' Speaking of dark sides, Wolverine must also unite frenemies Erik (Fassbender), who's been (wrongly?) imprisoned for the JFK assassination, and Charles (McAvoy), now a drugged-up recluse living with Hank, a.k.a. blue-furred Beast (Nicholas Hoult). Says McAvoy, ''Wolverine has to help me figure out who I really am, what I really want in life, and what I'm willing to sacrifice to get that.'' The film cuts back and forth in time, but the majority of Past takes place, appropriately, in the past.