I'm proud of myself, I think I got most of the references within the different periods and could follow the plot quite good. Of course, when they reveal the different roles in the end credits I was surprised like the rest of the audience (well, it's ok to miss the minor roles but Hugh Grant as the Kona chief caught me completely by surprise).
If nothing, Cloud Atlas should win the Oscar for Best Makeup, the technical aspects of the film are superb anyways. The whole production is visually stunning thanks to great set designs, costumes, effects, cinematography and make-up and prosthetics, not to forget the expert editing that uses many smart jump cuts from one period to another that convey themselves many symbols and recurring motives.

So, you could say I'm boisterous but I'll do my best with a synopsis, chronologically from the middle of the 19th century to post-apocalyptic future.

First in order is Adam Ewing (Jim Sturgess), an American lawyer who's visiting some Pacific islands (because of some slavery subplot) and gets slowly poisoned on the trip home. You could argue this is the least action-packed of the episodes but one, that first depicts two-class society which is the main theme in the 2144 piece thus sets the base for what I think, the Wachowskis tried to tell us.

Anyways, his memoirs are read by 1930s Robert Frobisher (Ben Whishaw) a young aspiring composer, a gay one to be precise which is the reason why he shoots himself after composing his opus magnum (which is the Cloud Atlas Sextet). He's also the lover of Rufus Sixsmith (James D'Arcy), who, forty years later, works a nuclear scientist with a dark secret which he reveals to Luisa Rey (Halle Berry), a journalist, who gets in the firing line of oil lobbyists.

Her friend Javier Gomez (Brody Nicholas Lee) writes a book about her investigations, that is read by publisher Timothy Cavendish (Jim Broadbent) while fleeing shady gangsters who want to press money out of him (concerning another subplot centering around an atypical Tom Hanks performance).
Cavendish is tricked by his brother into moving into a nursing home where he is tyrannized by the staff (especially by Hugo Weaving's Nurse Noakes, he plays the villain in all pieces btw). He later writes a film script about his "ordeal" which gets filmed with another Tom Hanks in the leading role.

This film triggers an idea in the mind of replicant Sonmi-451 (Doona Bae), which connects the 1849 episode with this one set in 2144. Sonmi decides that all people should be treated equally, both the clones and the "real" ones and records a video speech, which, years later (in 2321), evolved into the religion of the tribal people on post-apocalyptic earth that has Zachry (Tom Hanks) meet one of the "Prescients" (Halle Berry) who wants to contact the colonies on outer space through a radio station that is located on the holy mountain of the tribesmen. Zachry works as a guide for her and... - can't really write more about it without spoiling it.

In the prologue and epilogue we see an older version of Zachry who tells these stories.

You maybe don't believe me but writing this was actually good fun :)

Great film, almost impossible to grasp with one viewing. You'll either hate or love it.