At the beginning of Episode 4, Bell encounters a mermaid named Marie, who had cheered his victory over the Iguazu. Marie offers Bell her healing blood, revealing her kind nature and her connection to Xenos like Rei and Rido. Contrary to initial assumptions, Marie is not allied with the Moss Huge but is driven by her loneliness due to her aquatic limitations.

Back in Bell's group, Lili and Aisha take charge in Bell's absence. They find themselves in a tough situation as the Moss Giant manipulates the monsters' movements, leading to a "pass parade" where hordes of monsters attack from all directions. Despite being overwhelmingly outnumbered, the group manages to hold their ground. Welf makes extensive use of his magic swords, Ouka showcases his axe skills, and Makoto demonstrates a new technique to effectively kill a giant sea urchin.

Meanwhile, Bell and Marie form an alliance. Marie uses her song to locate a friendly monster and guides Bell back to his group. When Marie can no longer swim, Bell carries her, fulfilling the series' titular premise.

Despite small victories and time for Cassandra's recovery, the group continues to face relentless waves of monsters. Lili recognizes the need to reduce the number of monsters, and Aisha suggests utilizing Haruhime's level boost for the entire group.

The episode ends with the anticipation of Bell and Marie's arrival to support the group, which has shown remarkable endurance and coordination in Bell's absence.