There's actually more than one Ant-Man in the film, as we start with the original, Dr. Hank Pym, a leading scientist, and inventor of the "Pym Particle", which allows things, including people, to shrink own to ant-size, or more, but with enhanced strength. Through a series of ingenious plays, Pym manages to get his Ant Suit into the hands of resourceful (but jobless) cat burglar Scott Lang, recently released from prison for theft on a corporate scale, though he did it to return money the company reportedly bilked form its customers. The reason for the ruse: Pym wants Lang to thwart the plans of his power-hungry former protégé Darren Cross in using his designs to create his own method shrink people down to miniature size in the form of the flying, weaponized Yellowjacket suits, which he aims to do for nefarious purposes. Pym's shrewd and scrappy daughter Hope still works in the company under Cross, which gives them the inside knowledge they need for Lang to breach Cross's tight security and stop the Yellowjacket project before it disrupts global warfare as we know it.