Tony Stark (and Robert Downey Jr., who portrays him) is the heart of the Marvel Cinematic Universe, and the tech billionaire has made smaller and smaller appearances in the universe since Age of Ultron.
Before trying to determine where Iron Man is prior to Avengers: Infinity War, it's prudent to go back to the beginning to see where Stark's been and how he's become the man he is. The MCU is a rich saga, and it all began with Iron Man in 2008. From those humble beginnings it has grown into a tale of cosmic proportions, with one of the biggest bads of all knocking on the door of the Avengers as the impossibly strong Thanos (Josh Brolin) finally invades Earth.
Through all of this, Stark has evolved drastically as as person, yet he still possesses fundamental flaws that have profound effects on the world. Fans have been speculating as to who might die in Infinity War, and Stark's massive ego could cost him his life. Stark fans can only hope he gets it together so he can be a part of the MCU's Phase Four.
Stark starts out as a billionaire playboy who is happy to profit off of war. That all changes when he is abducted by a terrorist group known as the Ten Rings, and sees firsthand his own weapons being used to kill innocent civilians.
It is there Stark forges a new heart - both literally and figuratively - creating the first miniature arc reactor capable of powering what would come to be known as the Mark I Iron Man armor, which he made in a cave out of spare parts.
Starks's mettle is tested even further when he learns his longtime business partner and surrogate father figure, Obadiah Stane (Jeff Bridges), is behind his abduction, and he's forced to kill Stane in a final confrontation. The movie closes with Stark proclaiming himself Iron Man to the world.
Stark manages to keep the shrapnel in his bloodstream from reaching his heart with the miniature arc reactor embedded in his chest. Unfortunately, the reactor is killing him, poisoning him so he appears doomed with or without it.
Fortunately, Nick Fury (Samuel L. Jackson) intervenes, and that's when Stark is first introduced to S.H.I.E.L.D. Faced with his impending death, Stark falls pretty far and sinks into alcoholism, but Fury gives him footage of his deceased and historically distant father that shows a different side of Howard Stark.
Despite his outward coldness, Howard Stark loved his son, and predicted Tony would create a brighter future. Stark begins to make a turnaround, re-evaluating his father's old work, only to discover a new element he uses to power the arc reactor, thus solving the problem of the poisoning.
Ivan Vanko (Mickey Rourke) believes Howard Stark stole his father's work years ago, and thus vows revenge on the Starks. Seeing Stark's arc reactor on TV, Vanko builds his own, ultimately becoming the villain Whiplash.
Meanwhile, Stark has already built a couple suits, and as he struggles through his internal battle, his longtime friend James Rhodes (Don Cheadle) confiscates the Mark II armor and retrofits it to create War Machine. Iron Man and War Machine team up in the end of the film to defeat Whiplash, beginning a longtime heroic partnership.
In the first team-up, the Avengers prove to be a relatively volatile crew, a "time bomb" as Bruce Banner (Mark Ruffalo) puts it. At the heart of this time bomb is a brash and arrogant Stark.
While the team manages to ultimately come together and stop Loki (Tom Hiddleston) from conquering the planet with his invading Chitauri army, the team still gets off to an inauspicious start - not only did they assist in the mass destruction of New York City, but they also forge a tentative but strained alliance marred by mistrust.