The thing about British serial killers in the movies is that they’re so terribly polite. Watching “Sightseers,” a low-budget lark about a homicidal couple touring the backcountry, one is reminded of Alec Guinness drolly dispatching his relatives in “Kind Hearts and Coronets” (1949) or poor, sweet Michael Palin forever murdering the wrong dog in “A Fish Called Wanda” (1988). No matter how many brains get bashed or how much blood gets spilled, it’s always important to mind one’s manners.
Chris and Tina (Steve Oram and Alice Lowe, who collaborated on the script with Amy Jump) are a sad-sack couple who’ve been dating for a few months when Chris arrives in his caravan camper-van to take Tina on holiday. The byroads and tourist traps of the Lake Country, the West Midlands, and Derbyshire are calling, drowning out the needy whine of Tina’s mother (Eileen Davies), who still hasn’t forgiven her daughter for impaling the family dog on a knitting needle. (“It was an accident, Mum!” “So were you.”)