And what happened to Sam? Well, he's busy being throttled by Lucifer - trying to convince Sam that he's still his "little bitch" in Hell. When Dean appears on the scene, Lucifer disappears and Sam joins his brother in pursuit of Cas.
But Cas is in a bad state - he wades into a lake, with Dean, Sam and Bobby arriving just in time to watch him disappear beneath the surface. The lake is transformed into a huge whirlpool, then turns jet-black, and then nothing - the Leviathans are gone.
"If they're in the pipes, they got themselves a highway to anywhere," notes Bobby. Moments later, Dean discovers Castiel's trademark trench-coat floating in the lake - it looks like, this time, he really is gone for good.
As Bobby predicted, the Leviathans spread through the water, infecting a little girl and a mechanic named Edgar. But right now, Dean and Bobby are more concerned with Sam's mental well-being. "How you doing?" Dean asks his brother. "And do not say OK!"
Sam admits that he is still having visions of Lucifer and that he is beginning to doubt if he ever left the Cage. "You know that he's not real, right?" asks Dean, but Sam replies, "He says the same thing about you."
It's all getting too much for Dean, but the stoic elder Winchester won't admit to Bobby that he's feeling the strain. Bobby reminds Dean that he just lost one of his best friends and his brother is losing his mind - he's definitely not fine, and any claims otherwise are "utter horse-crap".
Meanwhile, at a local high school, two high-school athletes have been possessed by the Leviathans. Desperately hungry, the creatures slaughter and devour their fellow students. And at the nearby Sioux Falls hospital, Bobby's old ally Sheriff Mills is recovering from a routine appendectomy - let's hope she won't get involved in any trouble...
When the brothers and Bobby stumble across police reports relating to the massacred high-schoolers, Dean sets off to investigate, leaving Bobby to keep an eye on the troubled Sam.
Elsewhere, the possessed Edgar meets with the little girl Leviathan in a local park. He's angry about the high-school deaths - it has attracted too much attention. But the little girl Leviathan has a plan - soon the creatures will be able feast in peace....
At Sioux Falls, the charming Dr Gaines finds the Leviathan girl and is taken in by her sweet and innocent demeanour. But the creature's true colours are soon revealed - she devours Gaines and assumes his physical form.
Posing as a federal agent, Dean investigates the high-school massacre and spots generous helpings of black goo - a sure sign that the Leviathans are involved. Back at the hospital, the creatures have moved on to their next victim. Sheriff Mills is stunned when she discovers Gaines dismembering one of his patients and eating the victim's liver - yuck!
Back at Bobby's yard, Sam's struggle with the devil inside him continues. Lucifer tells him that his torture will never end - he's enjoying it far too much. There's only one way out for Sam - "It ends when you can't take it any more." Is suicide Sam's only option?
A frantic Sheriff Mills calls Bobby and blurts out five words that he did not expect to hear - "My surgeon is a monster!" Bobby heads out to rescue Mills from Sioux Falls, leaving Sam alone.
Edgar and the two high-school athletes meet with Gaines at the hospital and are led to a back-room, where the head nurse and staff administrator are held captive. Gaines explains that the hospital is a perfect feeding ground - no-one will question a few unexpected deaths. "It's exactly what the boss asked for," beams Edgar, as the possessed teens feed on the hostages.
As Bobby rescues Mills from the hospital ("Bobby Singer, my hero," she gasps, "That's the roofies talking," he responds), Dean returns to Bobby's yard and asks for Sam's help. He knows that his brother is "bonkers" but insists that they have to work together to track the Leviathans.
Sam and Dean depart, but moments later, the real Dean returns to the yard - the Dean accompanying Sam is an impostor! As they arrive at a deserted warehouse, the fake Dean transforms into Lucifer and again tells Sam that there's only one way out - he must take his own life.
Back at the hospital, Bobby is confronted by the possessed Gaines. "We saw you," he tells the hunter. "Through the angel's eyes." Bobby fires a buckshot of silver straight into the Leviathan's chest, but it has no effect.
Meanwhile, the real Dean tracks down Sam, but finds himself staring down the barrel of a gun. Sam is deeply disturbed and doesn't know who to believe - Dean or Lucifer. But, lest we forget, Dean has been to Hell too - he tells Sam that suffering down there is a different kind of pain, unlike what you experience on Earth.
Last episode Sam fell on broken glass, injuring his hand. Dean grabs his brother's "gimp hand" and squeezes, hard. His plan seems to work - feeling the intense pain, Sam knows which world is real. Lucifer begins to flicker away and eventually disappears entirely.
Have escaped the hospital, Bobby calls Sam - the Leviathans have taken over and he's heading back to the yard to regroup. Dean and an apparently recovered Sam head in Bobby's direction, but when they arrive, they find the yard a burning wreck - and there's no sign of Bobby!
As the Winchesters frantically search for their friend, Edgar appears. "You two are competent enough to warrant annihilating," he tells them. "I'd take it as a compliment." Dean shoots the villain straight in the face, but it does no good. Edgar violently throws Dean into a nearby vehicle, breaking his leg.
Edgar sets his sights on Sam, beating him around the head with an iron bar. Even crippled, Dean's not out of the game - he flattens the Leviathan with a car. Unable to move and with Bobby still missing, Dean is forced to call for an ambulance.
When Sam wakes on the way to hospital, he has another vision of Lucifer. "So maybe I'm not real - nobody's perfect," he grins. "But I'm not going anywhere, Sam."
Sam begins to have a seizure, but there's more bad news in store - the medic tells Dean that they're heading straight to Sioux Falls - the new home of the Leviathans! And back at the yard, Edgar's fingers begin to twitch - he's not dead yet...
• 'Hello, Cruel World' is a solid Supernatural episode, but we're hoping that the Leviathans aren't a season-long villain - they don't match up to some of the classic baddies from seasons past.
• And is that really the last we've seen of Castiel? Misha Collins better be back - his rudimentary send-off five minutes into this episode was hardly a fitting exit for such a major character.
• Is the apparently alive Edgar going to be a recurring villain then? We've been a fan of actor Benito Martinez since his time on The Shield so here's hoping!
• Got to love Dean and Bobby's manly bonding moments - Bobby clearly cares deeply about his surrogate son, but can't express it without using the word "Idjit".
• One thing we will say for this episode - what a cliffhanger! Sam on the brink of death, Dean badly injured, Bobby missing - it's going to be a long wait until ep three, 'The Girl Next Door'!