The scientist Dr. Henry Pym developed a serum, which can cause rapid shrinking and equally rapid growth. He tested the substance on himself and is overwhelmed by the scale. To control the forces, he developed a helmet with which he can change his size at will. Ant-man is born.
In the film, the very controversial in the comics Henry Pym will assume only a supporting role. In the Centre of the film to the Ant man the thief will be instead Scott long, who steals Pyms shrinkage suit and as a result becomes a better burglar. But he is not out of selfishness, but to those he loves, to protect.