Sturgess pauses. ''Yes,'' he replies, ''but what is an ocean but a multitude of drops?''

This is the substance of Cloud Atlas, a huge epic that mingles six stories that range from the 1849 shipboard drama to an end-of-days adventure set on a devastated Earth. From one story to the next, characters struggle towards finding out truths or freeing themselves from oppression. ''It is a beautiful thought, that decisions we take and actions we do have consequences,'' one of the film's co-directors, Tom Tykwer, says. ''But we are not in a position to judge how big or small those consequences are, which gives our deeds more gravitas and more relevance. We have a responsibility with everything we do.''

Cloud Atlas is based on the Booker-shortlisted novel of the same name by David Mitchell, who adapted it along with the three directors, Andy and Lana Wachowski - who also directed the Matrix films - and Tykwer, whose earlier films include Run Lola Run and Heaven.

The Wachowskis and Tykwer divided direction of the stories between them, Tykwer taking the more naturalistic ones: of a young gay composer working for a mentor in 1936, a crusading journalist fighting plans for a nuclear reactor in 1973 and the disappointments of an ageing London publisher in 2012.

Sci-fi supremos, the Wachowskis handled the voyage, an extraordinary view of a future dystopia called Neo-Seoul in 2144 and the 24th-century apocalypse.

These stories are told sequentially in the novel. The film cuts them all together, with some scenes running only for a few seconds, to create a vast mosaic in which each story echoes within every other story.

The fact that the star-studded cast - which includes Tom Hanks, Halle Berry, Hugo Weaving and Hugh Grant - appears in different guises in each story emphasises this cross-weave of character and theme; it is up to the viewer whether to treat their reappearances as evidence of the transmigration of souls, as a twist on modern genetics or merely as metaphor.

Tykwer isn't a reincarnation man, but is sure we're not merely the sum of our socialisation. ''I had a son born a few years ago,'' he says.

''I looked at him coming out and he looked at me and I knew there was somebody already there; I could really see a character and it was proven right: he is that character now.''

He has no idea where that character was formed; perhaps it was in some parallel life. ''But one of the beautiful things about the novel is that you can both read it as a spiritual or secular tale,'' he says.

''It allows every perspective on it. Some actors I would ask, because it seemed appropriate, 'Where are we now in your soul's development?' Other days you would say, 'Inside your genetic string, what is your situation?'''

So there is a suggestion that the characters are evolving or devolving through a succession of lives and encounters with kindred spirits.

''The movie in a way is an investigation of progress as a subject in general related to humanity,'' Tykwer says. ''Is progress happening? Is it even a possibility? We are so … gifted, such amazing scientists, we can build the most incredible bomb imaginable, but then in the end we can't resist throwing it at ourselves. No other species would ever do that.''

But no other species can imagine itself, either. In that sense, the wild, tumbling fantasy of Cloud Atlas is absolutely human.

CLOUD ATLAS

GENRE Sci-fi/drama.

CRITICAL BUZZ ''As inventive narratives go, there's outside the box, and then there's pioneering another dimension entirely,'' wrote Variety when the film screened in Toronto, predicting a ''substantial audience … assuming critics don't kill it in the cradle''. It seems they did; Cloud Atlas has been overlooked in almost all the awards lists.

STARS Tom Hanks, Halle Berry, Jim Sturgess, Hugo Weaving, Susan Sarandon and Hugh Grant.

DIRECTORS Andy and Lana Wachowski and Tom Tykwer.

RATED MA15+.

RELEASE Now screening.