Tip: See my list of the 
MostIt’s easy to make a mistake in this one. If you want to say “if something happens (by chance), then …”, you should use “in case of”, without the article:
correct In case of fire, call the fire department.
wrong In the case of fire, call the fire department.
correct In case of rain, we will come back home. (correct)
unnatural In the case of rain, we will come back home. (unnatural)
“In case” can be used in the same meaning also without “of” in the phrase “just in case” which means “if something understood from the context happens”:
It’s dangerous outside. I’ll take my gun with me, just in case.
Note also that “in case something happens” (without “of”) means “because something might happen”, for example:
I’ll take an umbrella in case it rains.
The phrase “in the case of” exists as well, and it is usually used in the meaning “in the matter of”, “in relation to”:
correct I know that you have always been faithful, but in the case of your husband, I wouldn’t be so sure.
wrong I know that you have always been faithful, but in case of your husband, I wouldn’t be so sure.
