The theory of Karma states two things for a fact:

1) I did what I did in the past and my present context is a product of my past karma and I have no control over it.

2) I have complete choice over how I respond to my present circumstances and my future context depends on my present response. So I have complete sway over my future.

To sum it up, I have no control over my present external situation which comprises of my state of health, relationships and other circumstances but if do not react and rather responsibly act in the present, I can design a future of my choice.

Just visualise a group dance that one has to perform on stage. The person has rehearsed his steps and is required to match his steps with the music being played. This is all that is required to ensure a good performance but the dance is being performed in a group. There is the illusion that we are dancing together and we must match our steps with each other. The truth is each one has to match his step only with the music and if each one does that, we will all be automatically matching our steps with each other. But often we fall into the trap of deception and start matching our steps with each other and if one goes wrong, others follow. However, even while dancing together, if I stay in awareness that I just need to perform according to my director’s instructions and if I do so, my performance will be fine, then I do not get affected by what the other is doing. 

In life as well, all we need to remember is I have to stay in awareness and do good karma, despite everything. The Director(God)  I do not have to let others’ behaviour or the situation deceive me into performing wrong action because the law says I shall reap only as I sow. Reaction makes us sow the wrong seeds. Responsible action ensures that I sow the right seeds despite everything. If I sow the right seeds, I need not worry about the fruit. The law ensures that the fruit is good.