The structure 万一…，就…
"In case..., then..."
The structure 万一…就… 
expresses the result of an intended action or situation. It usually has a negative connotation.
万一 is followed by an intended action or situation.
就 is followed by the result of the action or a solution to the intended situation.
expresses the result of an intended action or situation. It usually has a negative connotation.
万一 is followed by an intended action or situation.
就 is followed by the result of the action or a solution to the intended situation.Scheme
万一
intended action
就
result
If a sentence has one subject, the subject is placed before 就 in the second part of the sentence.
If a sentence has two subjects, the first subject is placed after 万一 and the second subject is placed before 就.
Scheme
万一
(subject 1)
predicate 1
subject 2
就
predicate 2
Examples
万一他不来，我们就自己处理
If he doesn't come, we'll deal with it ourselves
wànyī tā bù lái, wǒmen jiù zìjǐ chǔlǐ
万一出事，你就怎么办？
What will you do if something goes wrong?
wànyī chūshì, nǐ jiù zěnme bàn
In this structure, 就 can be omitted. 的话 can be placed at the end of the first part of a sentence.
can be placed at the end of the first part of a sentence.Scheme
万一
intended action
的话
(就)
result
Examples
万一你需要帮忙的话，这是我的微信
If you need any help, here's my WeChat
wàn yī nǐ xūyào bāngmáng de huà, zhè shì wǒ de wéixìn
万一他不来接我的话，我就乘出租车回家吗？
Should I take a taxi home in case he doesn't pick me up?
wàn yī tā bù lái jiē wǒ de huà, wǒ jiù chéng chūzūchē huí jiā ma
你一定要带雨伞，万一下雨，你会湿透了
Take an umbrella in case it rains, or else you will get wet
nǐ yídìng yào dài yǔsǎn, wàn yī xià yǔ, nǐ huì shītòu le