Ant-Man

Scott Lang, a burglar fresh out of prison, is recruited by Hank Pym to steal a shrinking suit before Pym's protégé Darren Cross can weaponise the technology. Lang succeeds, goes subatomic to sabotage Cross, and — critically — returns from the Quantum Realm, which Pym had believed impossible. That single fact is the seed of Endgame's entire solution.