Forgive me for what I'm about to do.
Synopsis
The universe experiences a dragon break while the Enterprise is in orbit around an arid planet. They beam down to find Todd Howard with a stringy goatee has crashed his concept car in the California desert.
They take the Toddhead back to the ship, where he oscillates between between being reasonable and crazy, and injuries keep appearing and disappearing on him. Yes, it's true: the Toddhead has divided himself into the primordial forces of Anu and Padomay. If they should ever meet, the universe will be destroyed.
Anu (good Todd) steals dilithium crystals in order to power his car back up. Padomay (evil Todd) also steals dilithium crystals, also to power his car back up. As far as I can tell through the messy storytelling, Padomay is from the normal universe, and Anu is from an anti-universe. Which means there's also a car and an anti-car, which really sucks for dilithium crystal mileage.
Kirk gets sent to the anti-universe, where Anu tells him his plan: for Kirk to send Padomay into the corridor between worlds where the two halves of the Toddhead can fight forever. After pushing Padomay into the corridor, they blow up his car, which also destroys Anu's car for some reason. Spock remarks that now the universe is safe, but Kirk wistfully asks, "What of the Toddhead? What of the Toddhead?"
Commentary
"Mirror, Mirror" is episode 2x10. Let's just, uh, hold out for that one.
Very random side note. Apparently the Toddhead was originally supposed to be played by John Drew Barrymore, father of the actor John Blythe Barrymore (and also some actress named Drew Barrymore). Appropriate enough, since I spent a fair bit of this episode wondering where my pipe could be.