And you thought Dexter couldn’t find darker places to go?

Some of the best television series use their fourth season for a radical shift in their basic premises. Joss Whedon sent Buffy off to college in her fourth year, ending the series’ long-running conceit of the high school student fighting the forces of darkness. David Chase let the marriage of Tony and Carmella deteriorate into a separation in the fourth season of The Sopranos. This set up a fifth season in which the cross-grained tensions of Tony’s two “families would be fully exposed.

Dexter is following this narrative trajectory, with a fourth season now available on DVD and Blu-Ray that allows his dark passenger to travel in unfamiliar territory. Married now to too sweet-to-be-true Rita (Julie Benz) the quiet loner with a penchant for power saws has to navigate the responsibilities of wife, children and a life in the suburbs. America’s most beloved serial killer must trade in his nighttime activities for a daylight world and the results are close to disastrous for our anti-hero.

Along with these cataclysmic changes to Dexter’s personal life, season four introduces us to the series’ consummate villain, the Trinity killer portrayed in utterly creepy glory by John Lithgow. Lithgow constructs a frighteningly normal character that also happens to bury, drug, slice and bludgeon his victims in an operatic re-enactment of his own family tragedy. He becomes the Joker to Dexter’s Batman, a funhouse mirror of Dexter’s own psychosis.

One of the important elements of season four is how it radically calls into question our empathy for Dexter and the moral casuistry required to make a serial killer into a beloved character. In Season Two, the moral quandaries of Dexter’s code that only allows him to slaughter the guilty are resolved for him as Doakes, the cop on his trail, is killed by the other villain of the season, Lila. This allowed a neat resolution in which Dexter could then off her, since her murder of Dexter’s other nemesis had made her suitably guilty. The trash gets taken out and Dexter has stayed within his bushido-like code.

No such absolution came for Dexter in Season Four as he faces the possibility of having killed an innocent. Moreover, he is tentative about getting rid of the Trinity Killer, seeing in him almost a kind of mentor. As an audience, we wait impatiently for him to rid the world of Lithgow’s vicious character and Dexter’s refusal to do so is presented as a monster’s sympathy (admiration?) for another monster.

Dexter as a series has continued to mature and the writing has continued to be strong. Even more importantly, it seems to me that our relationship to Dexter has continued to change. Consider how much we depended on Dexter for the primary narrative in season one, his voice explicating, justifying and giving meaning to every event and person. This seductive narrative style has continued and we mainly continue to experience Dexter’s world with Dexter’s guidance.

Season four, however, forces the audience to consider this fact more closely and to interrogate it. The apparition of Harry, Dexter’s stepfather who recognized his homicidal tendencies and taught him “the Code”, provides a voice that contradicts Dexter’s own inner narrative. Lithgow’s Trinity killer does much the same, showing us another serial killer with what seem to be some empathetic characteristics and who even, like Dexter, manages to have a normal family life.

Or so it seems early in the season. Of course, Dexter remains the dark hero of the piece and in the season’s best episode (I would argue the best episode of the entire series) the depths of the Trinity Killer’s evil is revealed at a Thanksgiving Dinner from Hell. In the dénouement, both Lithgow and Michael C. Hall delve into the darkest corners of their own psyches to unleash a performance that will give you nightmares.

Dexter Season Four has upped the ante for its audience, utterly transforming the main characters’ world while maintaining the basic moral tensions that make it work as both a drama and a meditative reflection on the nature of violence and evil. By the last episode, it’s our own dark passengers that are called to account.