The new villain is Tom, a classic vigilante killer who gets the drop on Luther, but lets him go, because he doesn't want to kill him. After all, as Tom tells Luther, they're on the same side, both hunting bad guys. But Luther can't accept that, because he's just a cop, he says, not judge and jury.
Now actually, as we know from the past two seasons, and even this one, that's not completely true. When sufficiently enraged, Luther's always ready to mete out severe corporal punishment. In that sense, Luther is a British Jack Bauer, though his quarry are less apocalyptic, and Luther hasn't (yet) killed his boss for the greater good.
But Luther is an idealist, which leads to his sometimes going overboard in trying to bring the wrongdoers to justice, and also causes him to sometimes have too much faith in good people responding to their better angels. His attempt, tonight, to get a woman who was raped as a girl by Tom's hostage - to get Tom to free the rapist rather than kill him - was bound to fail. Put on television to plead with Tom from afar for the rapist's life, she instead implores Tom to kill her rapist. Who can blame her?
And, so, when Luther and Ripley - who have now thoroughly reconciled - come to the scene to stop Tom from hanging the rapist, you just know it's not going to turn out well. But it turns out far worse than that.
Luther literally saves the rapist from hanging by holding him so the rope that Tom has left the rapist hanging from doesn't take its toll. He does this in an act of extraordinary courage, battling off the crowd who are punching him and kicking him in an attempt to dislodge him from the rapist.
Ripley, on Luther's instruction, is off in pursuit of Tom. British police aren't usually armed, so Tom gets the drop on Ripley, as he previously had on Luther. Tom pleads with Ripley to walk away. Ripley says he can't. And Tom responds by shooting Ripley dead. He's not as lucky as Luther, though luck is the last thing you could say Luther had in this story tonight.
And there this dynamite of a television episode might have ended, with Tom running away, and Luther left grieving by Ripley's body. But there's one more kick in the stomach left. Mary the blonde, waiting for Luther in his flat, sees a man standing out in the street outside Luther's window. It's Tom, come to exact some further retribution from Luther, for not accepting Tom's request not to make Tom his enemy. And as a grace note to this peril, we hear a few bars of Robert Plant's "Satan, Your Kingdom Must Come Down" ... that hauntingly evocative song from Boss. (This is apparently the season for series borrowing songs from other series, with Dexter playing "Make Your Own Kind of Music" of Lost fame.) To the sicko murdering vigilante Tom, Luther has become Satan.
We'll learn how this plays out tomorrow night in the concluding episode of this short wallop of third season.
See also Luther: Between the Wire and the Shield ... Luther 3.1: Into the Blender ... Luther 3.2: Success