、。
If by any chance it rains tomorrow, they probably won't go.
Literal
By-chance tomorrow rain [subject-が] falls-[conditional-ば], they [topic-は] won't-go [probably-だろう].
A textbook paradigm of the unlikely hypothetical: 万一 ('one in ten thousand, just in case') + ~ば conditional + ~だろう ('probably'). 万一 is heavier than たら/ば alone — it flags the condition as a remote possibility being entertained 'just to be safe.' Useful for warnings, contingencies, and cautious planning.