The Ending Of Ant-Man Explained
Despite what you've seen on the big screen, in the comics, Ant-Man started out as one of the original Avengers way back in 1963. He wasn't any more or less ridiculous than the guy who got big when he was angry or the guy with a limp who turned into a Norse god and spoke in Ye Olde English or the Howard Hughes knockoff cosplaying as Ned Kelly. He even beat the Hulk by making some ants dig a hole under him during a performance as a super-strong juggling circus clown. Absolutely none of this is made up.
And yet, somehow, the world never quite glommed onto Ant-Man the way that it did with other heroes, making his first solo outing a nail-biter of a moment for MCU fans. Sure, the studio had pretty much proven that they could print money on the backs of unknown characters with 2014's Guardians of the Galaxy, but would crowds show up with the same gusto for Hank Pym, Earth's mightiest spousal abuser with the power, as Dan Aykroyd observed, "to really clean house on those other ants?"
So Ant-Man starts by putting Pym in the passenger seat, showing us a former government operative sent on secretive information reconnaissance and sabotage missions thanks to his having developed the ability to get real small while retaining the strength of a full grown man. Upon learning that his proprietary super science is in the process of being replicated by S.H.I.E.L.D., he resigns from the organization and proclaims himself to be Ant-Man no more. This all happens in the '80s, though. You can probably forget about it.
Generations of Ant-Men collide
Meanwhile, in the present, Scott Lang (Paul Rudd) is having a rough go of things. Recently released from prison after pulling a Robin Hood act, he just can't find a 9-to-5, making it next to impossible for him to spend time with his daughter, Cassie (Abby Ryder Fortson). He begrudgingly takes a job burgling a mansion with some friends, finding only an old motorcycle suit and helmet in the case. He tries the suit on, only to find himself in the same unenviable position as Wayne Szalinski and his honey three movies into their adventures. He has, quite unexpectedly, shrunk himself.
Lang tries to return the stolen shrink suit, only to be arrested on the spot. In jail, he's visited by the outfit's creator, Hank Pym (Michael Douglas), who set the whole thing up as a test to see if Scott could help him with a very important job. In a plot twist not scene since the first Iron Man movie seven years prior, Pym has learned that his old chrome-domed colleague is close to perfecting a weaponized suit of his own, with shrinking capabilities on par with the original Ant-Man getup. He wants Scott to use his already-honed skills as a thief in tandem with his Pym Particles in order to steal this Yellowjacket armor. Also, he can talk to ants.
Scott trains, both with Hank and his daughter Hope (Evangeline Lilly), who resents her father for his part in her mother's disappearance years earlier when she made the rookie mistake of getting too small. Janet van Dyne, in a flashback sequence, is seen activating Chekhov's Gun in the form of a Pym Particle Regulator, shrinking herself to the subatomic level and becoming trapped in the Quantum Realm, another dimension of reality which can only be reached by getting really, really tiny.
Yellowjacket gets swatted at the end of Ant-Man
After a quick warm-up heist at Avengers HQ and a friendly fight with the Falcon (Anthony Mackie), Scott gets set to throw a wrench in the Yellowjacket's gears. He and his comrades break into Pym Technologies, where they learn that Pym's old protege Darren Cross (Corey Stoll) intends to sell his shrink tech to HYDRA. Our heroes are briefly captured and all seems lost, but a thrilling escape sees them breaking free and imploding the building, albeit too late to stop a nefarious-looking HYDRA agent from escaping with a wad of Pym Particles.
Cross lives up to his name, getting a real mad on. Using his Yellowjacket suit, the shrink-maddened entrepreneur takes Scott's daughter hostage, goading the new Ant-Man into a visual spectacle of a battle. Scott, seeing no other way, disables his regulator and shrinks to subatomic size, making sure to sabotage Darren's suit on the way down and (probably) killing the poor baddy. Through perseverance and maybe, just maybe, some help from the long-missing Janet van Dyne, he escapes the Quantum Realm and returns to normal size, redeemed as a father and ready to start his own security company.
In the end, Scott is informed that the Avengers might need his help. Hope is given her own shrink suit, complete with dragonfly wings of the sort that really would have helped earlier in the movie. Also, fun fact: in the comics, Cassie gets straight up murdered by Doctor Doom, leading Scott to try to mobilize a paramilitary force of children to kill the masked despot in 2013's FF. For one reason or another, the money men at the MCU never get around to slaughtering a child onscreen and then sending Paul Rudd on a revenge ride accompanied by minors. But that's showbiz, baby.