Hook's past with Ursula isn't pretty.

On Sunday's Once Upon a Time (8/7c, ABC), the former pirate (Colin O'Donoghue) arranges a woodsy rendezvous with the aquatic Queen of Darkness (Merrin Dungey), but it's not what you think. Watch the sneak peek above to see how these old acquaintances get on.

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"We see in this episode that Hook and Ursula have some sort of history together," O'Donoghue tells TVGuide.com. "We find out that once Hook had helped Ursula try to run away from [her father] Poseidon, who was using her voice [as a lure] to run pirate ships into rocks. He very genuinely offers to help her, but as is always the way in Once Upon a Time, it doesn't go according to plan."

Unfortunately, whatever Hook once did is "something present-day Hook would regret doing," the actor added. "But you see that Hook is taking a page from Emma's book and is offering Ursula a chance at her happy ending in exchange for her telling him what exactly [Gold/Rumplestilskin] (Robert Carlyle) is really after."

Could this past act affect his relationship with Emma? Check out the rest of our interview with O'Donoghue to find out:

Will Emma (Jennifer Morrison) find out about this secret?
Colin O'Donoghue:
Yes, Emma finds out, but Hook is also trying to make amends and trying to be a better person. He's also doing it because he wants to stop Rumplestiltskin from pretty much killing the woman he loves and everybody he's grown fond of. As is kind of the way with Hook, he does some things slightly questionably to do what he thinks is the right thing. But Hook is struggling a little bit with trying to let go of the old pirate and trying to be a hero.

That is an interesting point. While Hook is struggling for redemption, the other traditional villains want to force The Author to give them their happy ending. Why is Hook on a different path?
O'Donoghue:
Maybe you'll find out later on in the season. There are reasons that Hook does things, but at the moment, Hook is kind of content. He's trying to make a go of it with Emma. He's the kind of person, once he decides on something, he does it whole-heartedly. We'll see that he cares for Emma and that he's given himself to that 100 percent. His main thing at the moment is to try to protect her and protect whatever it is they're beginning to form in their relationship.

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Would part of that protection of Emma extend to when the Queens of Darkness try to awaken her dark side?
O'Donoghue:
You know, Hook lived with darkness for such a long time. I think he will try to use his history with that to help her make the most informed decision.

Can you tease what else Hook and Emma will be facing this season?
O'Donoghue:
They're constantly tested. Storybrooke is a bit of a disaster of a town to live in if you just want to have a normal, quiet life. They definitely get tested. It's just difficult especially because they're facing such a huge, uphill task with the Queens of Darkness and the Dark One. Altogether, it's a pretty difficult thing. But they all try to band together and beat them. We'll see toward the end of the season if they'll succeed or not.

Will we also find out this season what else is in the past between Will Scarlett (Michael Socha) and Hook?
O'Donoghue:
Not so much, but you'll see little elements of what their relationship was. The last time they saw each other officially, they had a bit of an argument. It will be interesting to discover what exactly their problem with each other is.

Is there a chance of Hook getting his hand back again like when he went on the date with Emma?
O'Donoghue:
Not at the minute. That was my one opportunity to use my left hand. It was a bit freeing, but it was weirdly awkward because I'm so used to wearing that costume and having a hook. So I'd be doing a scene and I'll be holding my hand as if I were hanging onto the hook. So I had to catch myself a few times not to do that.

Once Upon a Time airs Sundays at 8/7c on ABC.