The theory of Karma is the very basis of Hindu philosophy. Some people subscribe to it and some don’t. But Hindu philosophy makes no compulsions. It does not force one to accept its view of life. Acceptance is left to the free will of people. And they will decide based on their own interpretation of life.
But Navneet ji has put it very briefly and correctly. The play of Karma involves many lifetimes (again, rebirth is fundamental to Hindu Philosophy). There are volumes written on types of karmas and its linkage to final salvation from the cycle of births and deaths.
Believers will believe. Others won’t. But the Karmic activities go on regardless.