There is a moment, toward the beginning of the first episode in BBC America's most excellent crime drama Broadchurch, when you realize this will be a ride through a murder whodunit like no other. • It comes as David Tennant — best known perhaps as one of the recent generation of actors to play British sci-fi legend Dr. Who — drags himself to the feet of a young boy lying along the beach of a tiny seaside town. Tennant is playing Detective Inspector Alec Hardy, a damaged crime solver from the big city who had come to the town of Broadchurch hoping to lie low after a high-profile murder investigation exploded in his face in a previous job."God, don't do this to me," he mumbles, as a crowd of locals begins to gather. Among all the police in this tiny town where gasoline siphoning and bicycle thefts top the usual crime statistics, only Hardy knows the hell that is coming. But the viewer can feel it, too.A good story is in the wind. And we can't wait to dive in.American audiences can delve deeply into Broadchurch's amazing story when the series, which aired in Britain this spring, debuts on BBC America on Aug. 7 at 10 p.m. But besides offering a compelling, impressive story, Broadchurch manages something another slow-moving murder mystery show, AMC's The Killing, has not:Stretching the solution of a single murder over the length of a TV season without losing the audience.The Killing has tried for three seasons to accomplish this feat, airing the penultimate episode of its third season at 9 tonight. This year's episodes have centered on a serial killer who has likely slaughtered a dozen young girls living as throwaway children on the street.Where Broadchurch's complex story draws viewers deeper, The Killing brings more distance the longer you watch. Traits that once seemed intriguing — the obsessive, OCD-like focus of Mireille Enos' Seattle detective Sarah Linden — become annoying. The constant rain and moody atmosphere move from menacing and mournful to relentlessly, predictably downbeat. Even the series' brightest spot this season, the contentious yet caring connection Joel Kinnaman's streetwise Detective Stephen Holder has with a young runaway named Bullet, was taken away last week, as she became the killer's last victim.These days, there's so much high-quality TV around, viewers embark on a new series grounded in an unspoken covenant: I'll give you my time, if you promise to pay off my close attention with a story worth the effort.The Killing has been found wanting in its two previous seasons on that score, primarily due to creator-showrunner Veena Sud's decision to move the resolution of their first murder case from the end of the first season to the middle of second. Viewers were forced to wait many more months to learn who killed working-class teen Rosie Larsen last year, and many decided the knowledge wasn't worth another go-round with the series.Unfortunately, judging a series on the first few episodes is like judging a book by the first few chapters; sometimes it's tough to know what you've got until you've seen a bit more.So here's a primer on how to judge quality TV shows, based on the differences between what works on Broadchurch and what doesn't on The Killing. These concepts could be the only rules saving you from a long time wasted tracking a show that's not worth the effort.Characters aren't caricatures. Here's where Broadchurch and The Killing compare most favorably. Tennant's Hardy seems a typically damaged loner cop — estranged daughter, scandal-ridden past, doesn't play well with others — until you learn why his old case went sideways. Olivia Colman plays his reluctant partner, Detective Sgt. Ellie Miller. Inexperienced enough to shed tears while informing the dead boy's parents of his demise (he was also her son's best friend), she has an empathy with the town that becomes a huge blind spot over the course of eight episodes.On The Killing, Holder and Linden's dynamic is unique, as Holder's jive-talking white boy shtick leavens Linden's sullen intensity. But last week Holder fell down a dark hole as his anger over Bullet's death pushed him into attacking a former partner, while Linden tried comforting him with words of support even she didn't believe.Red herrings don't last too long. It's a staple of whodunnit shows that you get characters incorrectly ID'ed as suspects. This was a problem for The Killing, which can spend long episodes chasing a false suspect, fatiguing the viewer. Broadchurch has its share of suspects as well, but sorting them adds to the plot's velocity.The story is packed with multiple meanings. Here is where Broadchurch soars. Before long, you realize the show is a treatise on the hidden secrets every small town holds, the relationships and the ability of journalism to both illuminate and obfuscate, sometimes in the same report.Best of all, as the story resolves, we see a trading of roles between Hardy and Miller; where once her connection to Broadchurch was seen as an advantage, the tables turn by the end.It is harder to see larger issues in The Killing, beyond the obvious notion this year that bad parenting and a ruthless urban culture have created a generation of throwaway kids hustling on the streets. The police department here is overburdened and seemingly filled with cynics hanging on for full retirement benefits, while the prison guards supervising a condemned man seem as frazzled as the inmates.Here's hoping the last two chapters redeem The Killing's moody, struggling story. Because right now, Broadchurch stands as the best TV series yet to stretch a single murder investigation over a season's worth of episodes, setting a standard other shows will be hard-pressed to match.