Wonder Man follows Simon Williams as he tries to hide his superpowers while looking for his big break in Hollywood. Along the way, he finds a friend—perhaps his first friend—in Trevor Slattery, who has been ordered by the Department of Damage Control to spy on Simon and bring them incriminating evidence.
They're both eventually cast in a remake of the classic "Wonder Man," and Trevor finally confesses to Simon what he's been (reluctantly) doing to earn his freedom. Losing control, Simon walks into an empty studio and unleashes his powers, flattening the building.
When the finale begins, the DODC is closing in, and Simon realises his dreams are close to being over. Trevor, however, makes things right by reprising his most infamous role as The Mandarin (first seen in 2013's Iron Man 3). Trevor is able to convince the world that he's simply been posing as "Trevor Slattery" and takes responsibility for the explosion.
Agent Cleary isn't buying it, but it's a win for the DODC, and he's happy to move on from Simon, for now, after arresting Trevor and sending him to the expensive Supermax Prison we first saw in She-Hulk: Attorney at Law.
Simon gets to live his dream as the star of "Wonder Man," and Trevor is switched out for Joe Pantoliano (referencing an earlier joke in the series). At the premiere, it's made clear to Simon that he can now do anything he wants in Hollywood, and we later find him in the desert befriending a guard called Chuck.
He says it's preparation for a future acting role. Having spent time with Chuck and his family, the guard sneaks him into the DODC prison, allowing the actor to see first-hand what he does for a living. They go their separate ways, and Chuck learns that a huge amount of money has been deposited into his bank account, meaning he can finally quit his dangerous job as a guard.
Simon, meanwhile, uses his powers to disable a security camera. We hear Cleary explain to his boss that, based on the tests they've done, Simon is capable of harnessing ionic energy, making him "an extraordinary threat...or asset."
The "Wonder Man" star arrives at Trevor's cell, tears off the bars, and makes it clear that he couldn't leave his friend to rot. After mastering his powers, likely as a result of finally accepting who he is, Simon flies out of the Supermax with Trevor in tow, and they blast off into the distance together.
That's where the series ends, but the stage is set for these two to go on the run together from the DODC and for Simon to finally accept that he can become one of Earth's Mightiest Heroes. When and where that could happen will likely depend on Wonder Man's success, but it's a touching, feel-good conclusion to this story that leaves the door open to Simon and Trevor returning down the line.