Undo git merge of master into feature

I have two branches: Master and Feature.

I made some changes to Master and wanted to copy them into Feature. So I merged Master into Feature. Apparently, that was the wrong thing to do because now prior changes to Feature appear in Master.

What I expected:
A - B - - - - E - G   [master]
     \         \
      C - D - - F - H [feature]

What it looks like now:
                      G   [master] 
                     /
A - B - C - D - E - F - H [feature]

Here’s the fun part: I merged Master into Feature twice before noticing the problem, and I’ve made additional commits to both branches. How do I undo the merges or make it so Master is unaffected by Feature’s commits?

Git Baby is a git and github fan, let's start git clone.