Warning! SPOILERS about Outlander season 7, episode 4, and the Outlander books ahead.
A crucial story in Outlander season 7, episode 4 saw Brianna and Roger struggling with Jemmy's lies in 1980, but his talks of mythical Scottish creatures hint at a central Outlander story in the book An Echo in the Bone. Between unexpectedly expensive renovations at Lallybroch and getting accustomed to their new lives so different from what they left in 1700s North Carolina, Roger and Brianna faced plenty of challenges in Outlander season 7, episode 4. However, among the most puzzling trials, Jemmy's imagination ranked high, especially as he displayed knowledge of Scottish folklore neither Roger nor Jamie ever explained to him.
Jemmy demonstrated a peculiar imagination in Outlander, always saying what he believed to be true, even when that left his parents and grandparents puzzled. Being exceptionally in touch with his ability to travel through time was instrumental in getting Jemmy to know that Mandy could, too, making him essential to get the Mackenzies to a future that could have helped Mandy. However, Jemmy's deceitfulness with the broken radio didn't help him when he tried to convince Roger and Brianna of his telling the truth about Nuckelavees in Outlander season 7, episode 4, but that might have been by design, considering who likely asked Jemmy for food while offering that explanation.
Brianna was hesitant to believe Jemmy when he talked of stealing food for Nuckelavees in Outlander season 7, episode 4, but Roger's ancestor Buck Mackenzie had to use some kind of explanation to convince Jemmy and Mandy to help him in the book An Echo in the Bone. As Geillis Duncan's illegitimate son with Dougal Mackenzie, Buck Mackenzie unknowingly inherited the ability to time travel from his mother. Having returned with Morag and their family to Scotland in 1782, resting against the stones at Craigh na Dun transported Buck 200 years into a future he didn't know, leading him to hide around Lallybroch to survive.
Despite their difficult past, as Buck tried having Roger hanged, An Echo in the Bone let the two build a different relationship. While Buck used Scottish folklore to get Jemmy to bring him food, he was only trying to survive in a foreign time. Buck's seemingly sincere wish to return home led him to rely on Roger, as he and Brianna actually knew more about time travel than him. Considering also how Roger was the only person Buck knew in what looked like an absurd and foreign time, it made sense to turn to him, even if Roger could have reasonably been the last person to willingly offer Buck help.
Buck's presence in 1980 Scotland also presents Roger with the chance to time travel again in An Echo in the Bone. Indeed, believing Rob Cameron had brought Jemmy through the stones at Craigh na Dun, Buck and Roger follow. The development not only marks another trip back in time for Roger but also introduces another Outlander time traveler in Jeremiah Walter Mackenzie, Roger's father who was temporarily stuck in 1739. In the end, despite their thorny past, Roger and Buck make a good team in Outlander's eighth book, managing to save Jerry Mackenzie and send him back to his time, and meet various important characters from Claire and Jamie's past.