It takes some time for the reader of Never Let Me Go, the novel by Kazuo Ishiguro, to piece together what the three protagonists, children living at a bucolic, if menacingly organized boarding school, are being prepared for. This kind of slow reveal is not the way of movie trailers, which rarely save the first twist — the twist of the premise — for later. That said, the promising, melancholy trailer for music-video director Mark Romanek’s adaptation of the novel does what it can to be vague. “Sometime around your third donation, your short life will be complete,” Happy Go Lucky’s great Sally Hawkins tells a roomful of children — three of whom grow up to be Carey Mulligan, Andrew Garfield, and Keira Knightley — in the trailer’s most overt mention of the movie’s chilling setup. Even with all this restraint, the trailer will put an incipient lump in your throat. There’s melancholy and angst to spare here, if, perhaps, more emphasis on a love triangle than readers of the book might expect. At this early stage, Never Let Me Go appears to be that rare thing: a non-disappointing literary adaptation.