If the underlying concept of Devadas had to be summed up into a single line, it could best be done by citing this line from the film: "Aina chavuki bratukuki friendship enti?"
Because the entire film, even though we realise it only later, explores the strange friendship between life and death. Except, both these come in human form through Deva, a don representing death, and Das, a doctor (MBBS, MS, gold medalist, as we are reminded again and again) representing life.
That might sound complicated, but Devadas showcases it all with an element of fun, or as they say ....