Story [subject-が] boring was [because-ので] they [topic-は] one-person again one-person [quotative-と] departed went.
一人また一人と ('one person, then another') is a literary pattern for gradual attrition. 立ち去っていった combines 立ち去る ('to leave') with ~ていく (moving away from the speaker). ~ので gives a polite causal connection.