The movie is more interesting before they arrive in Manchester to the degree it's given over to the paradoxically aimless purposefulness of survival in a world so broken down and depopulated the survivors can't even really start over. You're conscious of allegorical buddings, but you can postpone trying to arrange them coherently because it's still early and the situation is absorbing enough on its own terms (although there's a wobble when the bogus suspense of a flat tire all but kills a shadowy, pestilential scene in an automobile tunnel).

And Murphy and Harris make a wonderfully complementary pair. She's the fierce one, who believes that since the virus takes effect within 10-20 seconds you have to be able to kill even your closest companion in a heartbeat. Selena has let the catastrophe shape her so that to her survival is only about survival; there's nothing on the horizon. Harris holds a machete as if it were a natural extension of her arm, and has an unshowy ability to suggest the emotion that Selena has had to suppress to be able to do so. (She also has a wonderful voice that can display a full octave span in a single word.) Jim is the one who feels the need of other people, who doesn't want to leave anyone behind, and Murphy's deep-set blue eyes radiate sensitivity. (He looks like the young Anthony Hopkins, but he's suppler, not so freeze-dried.) The movie switches the traditional sex roles: she's tough but delicate underneath, and he's delicate but tough inside. This doesn't register as political correctness but gives the couple an updated sexual tingle in what is, after all, and despite unspoken reference to recent headline viruses (in an interview with RES Boyle cites Richard Preston's The Hot Zone, a popular science book about Ebola, as an inspiration), an old movie set-up of the Most Dangerous Game (1932) variety.

It doesn't feel that old, however, until the foursome reaches the military compound outside Manchester, run by Major Henry West (Christopher Eccleston) whose practicality in the situation verges on the psychotic. West keeps a black man, one of his soldiers now infected, chained by the neck like a dog: he says that the sufferer "tells" him that the undead are incapable of getting or producing food and will eventually tell him how long it takes them to starve to death. Which is pretty sensible, except West lacks the normal dismay at the suffering of another human. (It's more experimentation, with a human "chimp.") Then when West addresses what he wants done with the Selena and Hannah, the only women in the compound, the movie loses what it had going for it.

The early parts are a survivalist fantasy coming on the heels of a destruction and re-creation myth. The way Boyle and Garland treat the survivalist fantasy allows the actors to create specific characters out of potentially overburdened types, and the movie works through the details of what they'd encounter, how they'd deal with it. But once the situation at the camp goes Gothic, it's time to start analyzing the narrative tropes and the fun's pretty much over.