The story of the heroes of "Once Upon a Time" in the underworld will continue with the upcoming Season 5 episode 15, titled "The Brothers Jones." As the title suggests, fans will get to meet Captain Hook aka Killian Jones' (Colin O'Donoghue) dearly departed older brother Liam (Bernard Curry).

Although Liam has been mentioned and shown in Killian's past, this will be the first time he has an active role in the ongoing storyline. It will also be the first time Emma (Jennifer Morrison) gets to meet hm. In "The Brothers Jones," the couple talks to Liam hoping he will be able to help them find a way to defeat Hades (Greg Germann), having been stuck in the Underworld for so long.

Liam reveals a mysterious book contains "the key" to taking down Hades, and Emma believes the book could be Henry's (Jared S Gilmore) storybook. However, the book Liam speaks of is one that only the souls that have tried to overthrow Hades know of, and considering Hades is still the ruler of the Underworld, it is safe to assume anyone who has tried has failed.

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The rest of the group decides to go on a search for the book, while Henry begin his own secret mission. As fans know, Henry had received a proposition from Cruella (Victoria Smurfit) in "Labor of Love," wherein she said Henry had the power to save everyone by taking on his role as the Author. Despite the pen having been destroyed in his world, it lives on in the underworld, because its purpose was not met by Henry.

If he wishes to, Henry can use the pen, but as fans know, all magic comes at a price and we don't know how it will backfire on him. Meanwhile, Hades will continue trying to pit all of them against each other in order to make their visit to Underworld permanent.

Watch Season 5 episode 15 of "Once Upon a Time" at 8 p.m. (EST) Sunday, March 27. You can also live-stream "The Brothers Jones" via ABC Go.