Hook (Colin O'Donoghue) will get an unexpected visit from his brother Liam (Bernard Curry) in the next episode of "Once Upon a Time" season 5.

According to the synopsis for the episode titled "The Brothers Jones," Hook will get a surprise visit from Liam in the Underworld as he and the other Storybrooke heroes find a way to defeat Hades (Greg Germann).

Liam told Hook and Emma Swan (Jennifer Morrison) that those who attempted to overthrow Hades in the past mentioned a book that contains the secret to the Underworld ruler's downfall. Based on the sneak peek for the episode, Hook and Emma realize that Liam might be referring to the Underworld version of the Story Book that Henry Mills (Jared S. Gilmore) possesses in Storybrooke.

This will prompt Snow White (Ginnifer Goodwin), Prince Charming (Josh Dallas), Regina (Lana Parrilla), and Rumplestiltskin (Robert Carlyle) to go on an all-out search for the book that could defeat the Underworld tyrant.

Meanwhile, Hades will reportedly exert more effort to keep the Storybrooke heroes in the Underworld by pitting them against each other. This could buy him time to keep them from finding out his weaknesses that could eventually lead to his downfall.

Meanwhile, series co-creators Adam Horowitz and Eddy Kitsis teased in an interview with ET Online that viewers will learn whether Liam approves of Hook and Emma's relationship in the upcoming episode.

"We'll find out this weekend if Hook's brother is a fan of Captain Swan," Kitsis stated. Horowitz also shared his thoughts, saying, "Just because he's dead doesn't mean a big brother can't have opinions on his kid brother's love life."

The episode will also feature several flashback scenes showing how the bond between the Jones brothers was shaped while they were onboard a merchant ship in a dangerous stormy weather.

"Once Upon a Time" season 5 episode 15 airs Sunday, March 27, on ABC.