Based on Agatha Christie’s 1929 novel “The Seven Dials Mystery” and adapted for television by Chris Chibnall (of “Broadchurch” fame), Netflix ’s “Agatha Christie’s Seven Dials” is a solid murder-mystery offering a peek into 20th-century aristocracy, centered on a plucky antagonist. While the series isn’t as thrilling as viewers may hope, the engaging performances and several of the less trite twists and turns work well enough. “Agatha Christie’s Seven Dials” opens in Spain in 1920. A middle-