The Underworld is heating up on “Once Upon a Time”! Last week Emma (Jennifer Morrison) and the other heroes failed to find Hook (Colin O’Donoghue), however, the pirate is doing everything in his power to reunite with his love in episode 13 of the ABC drama.

Sunday’s installment, titled “Labor of Love,” kicks off with Hook in an underground prison. There are no bars keeping him inside, but a fellow female prisoner warns him not to attempt to escape.

“He’ll hunt you down,” she warns Hook.

Hook’s not afraid of Hades (Greg Germann), but the Lord of the Underworld is not who he has to worry about. It’s Hades’ pet three-headed dog, Cerberus, who is the real threat. He’s got teeth that can crush bones and eyes that can burn through your soul. However, Hook’s ready to face the beast if it means getting one step closer to Emma. He helps the female prisoner escape, telling her that he’ll distract Cerberus if she can find Emma and warn her.

As Hook remains a prisoner underground, Emma and the others are doing whatever they can above ground. Emma searches the woods and finds the prisoner that Hook helped free. She’s willing to show them the cave through which she escaped. Meanwhile Mary Margaret (Ginnifer Goodwin) and David (Josh Dallas) roam the graveyard. It’s there that Mary Margaret finds a gravestone belonging to Hercules (Jonathan Whitesell). If they can help him move on, then he can help them.

A flashback to the Enchanted Forest reveals that Snow White and Hercules were friends when they were kids, before Snow White became a skilled archer. In fact, he was the one that helped train her and make her into the hero she is today. Snow White was too afraid to rule the kingdom and was going to give up her power to Regina (Lana Parrilla). It wasn’t until Hercules encouraged her to fight back that she earned the respect and loyalty of her subjects.

Mary Margaret finds Hercules working the docks in “Bizarro Storybrooke.” Unlike Mary Margaret, Hercules hasn’t aged. But fortunately for the heroes, he’s still willing to lend a helping hand.

Elsewhere, Regina sends Robin (Sean Maguire) and Henry (Jared Gilmore) to Cora’s (Barbara Hershey) office to find maps of the town. Cora put a spell on the doors to prevent anyone from entering, but Henry and Robin find a way in through an air vent. Robin stands guard as Henry enters the room, but he’s not alone. Cruella (Victoria Smurfit) is inside waiting for him.

Cruella reveals that she wants to return to the land of the living, however, Henry tells her that her wish is not possible. But Cruella has knowledge that proves otherwise. As the Author, Henry has the ability to re-write her ending. And it just so happens that the magical pen he snapped in Storybrooke made its way down to the Underworld.

But why would Henry help Cruella? She tells him that if she returns to the land of the living then Emma will no longer be a murderer.

As Henry thinks over Cruella’s offer, the others head to the cave that the prisoner escaped from. Mary Margaret encourages Hercules to fight Cerberus, however, he’s not the same brave fighter she used to know. He runs once he gets sight of the three-headed beast. But Hercules has good reason to run. Hades appears and reveals that his nephew died trying to fight Cerberus the first time around.

The backstory is that Cerberus was the last labor he needed to complete before joining his father at Olympus. He managed to collect the medals for the 11 other labors, but couldn’t defeat the three-headed hellhound.

Having never met Hades before, the others aren’t scared of him. But he knows just what to say to change that. He warns Mary Margaret that he’ll go after her family if she interferes with his. Hades also delivers a bloody hook to Emma, promising that he’ll do much worse to her beloved.

Mary Margaret’s ready to give up after Hades’ threat, but Regina encourages her to keep fighting. They may not need Mary Margaret’s inspirational speeches right now, but Snow White’s skills would certainly be handy.

That’s all Mary Margaret needs to hear. She seeks out Hercules again to tell him about her new plan – they fight Cerberus together.

The good thing is that they don’t need to enter the cave again, because Cerberus is loose above ground. The heroes split up to look for the prisoner, who ran away in fear. Mary Margaret and Hercules find her near the fallen clock tower and bring her into the library. But the doors are not going to keep the beast out. Mary Margaret makes Hercules give the prisoner his dagger, and tells them that together they can defeat it.

And Mary Margaret’s right. Working as a team they attack each head and kill Cerberus. That means Hercules can finally move on. But it turns out that Cerberus wasn’t his only unfinished business. The prisoner who aided them is actually Megara (Kacey Rohl), a woman who Hercules met prior to his demise. He believed that he had rescued her from Cerberus, however, the beast killed her after it killed him. Now, thanks to his second chance in the Underworld, Hercules was finally able to save her.

Defeating Cerberus didn’t just change Hercules, it also changed Mary Margaret. After feeling like a hero again, Mary Margaret tells everyone that she wants to be Snow White again. And the Storybrooke heroes will certainly need Snow White, because Hades has something dark and twisted up his sleeves.

“At first I wanted your friends to leave,” Hades tells Hook. “Now I decided they caused too much damage.”

Hades informs the pirate that for every person they help cross over, he has to decide which one of his friends is going to stay in the Underworld.