Don't do this: evading polymorphism

I've got two classes, Base and Sub. I can freely make changes to Base. But Sub is written by a third party and I don't have privileges to make changes to it. Here's what they look like:

public class Base {
  public void prepareForMission() {
    loadUpInfantry();
    loadUpWeapons();
  }
}

public class Sub extends Base {
  public void prepareForMission() {
    super.prepareForMission();
    loadUpOxygen();
  }
}`</pre>Suppose that I need to change the behaviour for instances of `Base` without changing the behaviour of `Sub` or other subclasses. 

The horrible, ugly hack I came up with for this is to evade polymorphism like so:
<pre class="prettyprint">`public class Base {
  public void prepareForMission() {
    loadUpInfantry();
    loadUpWeapons();
    _if (getClass() == Base.class) {_
      loadUpSecretPlans();
    }
  }
}

Doesn't it make ya cringe? I'm quite ashamed of it.