The main plot of 'Sovereign' and 'Heartburn' manga? I think there's a mix-up happening here, because those are two completely separate series with zero connection. 'Sovereign' is a Korean webcomic, a fantasy epic with political intrigue where the protagonist starts from the absolute bottom—like, slave-level bottom—and claws his way up using ancient magic and sheer cunning. It's a power fantasy, but the kind where every victory feels earned and the world-building around the different factions and the 'System' is surprisingly intricate. The plot is basically his ruthless ascent in a brutal, class-based society.
'Heartburn,' on the other hand, is a romance manhwa. The plot centers on a woman who gets entangled with a celebrity chef after a one-night stand, leading to a lot of forced proximity, workplace tension, and the slow unraveling of their emotional baggage. It's a classic contemporary romance setup, focusing on personal wounds and healing through relationship dynamics. The main 'plot' is their push-and-pull, will-they-won't-they, with a side of career drama.
They're so different in genre and tone that mentioning them together usually means someone saw the titles listed somewhere and assumed they were a pair. Maybe the confusion comes from both being full-color webtoons? But that's about it. One's a strategic power climb in a magical world, the other is a spicy, emotionally charged kitchen romance. You'd pick one based on whether you want scheming nobles or simmering glances across a restaurant.