In short, The Joker gets hold of Bane’s Venom and its Venom-delivering device and turns himself into ‘roided-out Joker monster.
Batgirl is actually the main character of this episode, not The Batman. There’s a message to this episode which is…”Brains > Brawn.” There’s also a kinda-sorta nod to Frank Miller’s THE DARK KNIGHT RETURNS when Batgirl shows up to save Batman from a very perilous situation.
Overall, “Brawn” was a decent THE BATMAN episode.
One more thing, is it just me, but did this episodes Venoned-out Joker look a lot like the Venomed-out Joker from the ARKHAM ASYLUM video game? Inspiration? Perhaps! - Bill "Jett" Ramey