The skies opened and your prized leather bag drank a bucketload of rain. Now what? Well, it’s not ideal by any stretch, but it is possible to repair leather after being left in the rain - if you act quickly.
As soon as humanly possible, wipe off excess water with a clean, dry cloth. Then, for a jacket, hang it on a wide, padded hanger. For bags or shoes, stuff them with crumpled newspaper to help hold their shape. Do not use a hair dryer or other heat source to speed up the drying process or you’ll have even more damage to repair. (You also do not need to wait until it is completely dry; you actually want it to reach the point of slightly damp, but this may still take awhile.)
When the leather is still slightly damp (but not soaked), wipe it down again with a soft cloth to be sure the surface of the leather is clean before moving onto the next step. (Dirty leather is tough to repair well.)
Do not skip this step. It’s crucial to condition leather that has gotten wet because of the oils in the leather bind to the water molecules. As the water dries and evaporates, it draws out the oils with it. The leather’s loss of natural oils causes it to lose its supple quality and turn brittle. So, while the leather is still damp, but clean, apply a reasonably thick coat of leather conditioner to repair the damage done by lost oils. Massage the conditioner into the surface of the leather bag and leave it to dry for several hours or overnight. (And after you’ve taken care of the wet leather situation, you may want to apply a water and stain repellent.)