(Reuters) - For the third installment of the lucrative "Night at the Museum" series, the filmmakers wanted to make a film about saying goodbye.

As fate would have it, the sequel, entitled "Night at the Museum: Secret of the Tomb" would be comic legend Robin Williams' final appearance before the cameras, as well as the penultimate film featuring veteran nonagenarian actor Mickey Rooney, both of whom died in 2014.

The credits for "Secret of the Tomb," which opens in U.S. theaters on Friday, read "In loving memory of Mickey Rooney," and "For Robin Williams - the magic never ends."

In the latest installment, the plot of which might best be distilled as "Night at the British Museum", Stiller, his teenage son and the museum's come-to-life characters must travel to London to save the decaying magical tablet that animates the natural history museum's exhibits each night.

Back for a third go-round in the series, which has so far generated nearly $1 billion at the worldwide box office, are Ben Stiller as the night security guard, Owen Wilson and Steve Coogan as the diminutive cowboy Jedediah and Roman Centurion Octavius, museum head Ricky Gervais, and Williams as U.S. President Teddy Roosevelt.

Australian actress Rebel Wilson joins the franchise as a lovelorn British Museum security guard, along with Dan Stevens, of "Downton Abbey" fame, as a comically non-comprehending Sir Lancelot and Ben Kingsley as a politically incorrect Egyptian pharaoh.

Stiller does double duty, also appearing as museum caveman Laaa who embarks on an improbable romance with Wilson's lonely British security guard.

"He set a really high bar," said Stiller of Williams, paying tribute to the actor's kindness and generosity of spirit. In the course of making three films together, he said he never got over being a Williams fan, a sentiment echoed by director Shawn Levy.

And when Stiller's character and Williams as Roosevelt bid farewell to each other toward the film's end, it strikes a poignant note.

"This character (Roosevelt) has always stood firmly at the soul of this franchise," said Levy, who helmed all three films.

He was vague about the possibility of a fourth film.

"This movie is about letting go. It brings a certain closure for these characters," he told a news conference.