Three Worlds Collide by Eliezer Yudkowsky

In a universe where three vastly different species encounter each other for the first time, a human starship crew must navigate the complex moral and ethical dilemmas that arise from their interactions. The story explores the clash of ideologies and values as the humans, who are guided by a utilitarian philosophy, meet the Babyeaters, a species with a disturbing cultural practice, and the Super Happy People, who seek to eliminate all suffering. As tensions rise, the narrative delves into the challenges of understanding and coexisting with beings whose beliefs and practices are fundamentally alien, ultimately questioning the nature of morality and the potential for harmony in a diverse cosmos.