For Episode 3 of Series 3 the Doctor re-visits "New Earth" in the year 5 billion.
Even though the Doctor said he'd take Martha on "one trip" he decides one to the past and one to the future would be fair. He takes her to New New York - and Martha does call him out over taking her to the same places he took Rose. The Doctor also lies to her about Gallifrey when she asks if they can visit his home, but he had his reasons.
NNY isn't quite as spiffy as it was when he last visited. This time they land in the "Under City" - specifically "Pharmacytown" - which is basically a slum where drug dealers do their business in patches that do very specific things - like "Forget" or "Happy" or "Sleep."
Martha gets kidnapped there by a newlywed couple who need another adult to gain clearance for the "Express Lane" (i.e., Ride Share) on the freeway. The freeway is an underground tunnel between NNY and New New Jersey - and it's been jammed up in gridlock for 20 years!! The tunnel, of course, is full of toxic exhaust so nobody "gets out and walks" (never mind the 1000 foot fall to the bottom of the tunnel). The Doctor, of course, sets out to rescue Martha, and in the process discovers some giant crab creatures at the bottom of the tunnel - who are responsible for the people who disappear when they get into the Express Lane. Whether or not the crabs are just destroying the boxy hover-vans or if they're also eating the occupants isn't spelled out in the episode.
The cat nurse from the "New Earth" episode in Series 2 shows up again - she's reformed her ways from experimenting on people and has devoted her life to caring for the Face of Bo. He sensed the Doctors arrival and sent the nurse into the freeway tunnel to find him, which she does (packing a gun "in case of pirates" so she says). She beams the Doctor off the freeway just as he had almost reached Martha. They rematerialize in the Senate Temple - where everyone is long since dead! In the "Upper City" only the Face of Bo and his nurse survived a plague. A plague caused by an interaction with a drug patch called "Bliss." The last things the Senate did was seal off the Under City and quarantine the planet for 100 years. Nobody in the Under City had any idea that there were no people nor authorities anymore - they just sat in traffic for years and years on end. The plague is long since over - the virus burned itself out. But there isn't enough power to un-seal the Under City - until the Doctor does a bit of rewiring and the Face of Bo gives up his own last energy to free everyone. Not, of course, before he reveals the "secret" he said he would tell the Doctor when they would meet "one more, final time." The secret is (drum roll please) - the Doctor is NOT the last of the Time Lords! However, the Doctor tells Martha that he thinks the Face of Bo was mistaken. Some possibilities that spring to mind, if the Face of Bo is correct, would be 1. The Master (of course), for how often has he been presumed dead only to resurface? 2. The Rani - the Doctor and she clearly had some history, she's also a renegade and I think was an unlikely recruit for the Time War with the Daleks, plus she's probably regenerated so they can easily recast the role. 3. Romana - which depends on whether you take the canonical slant that she was still in the dimension between N-Space and E-Space during the Time War, or if you take the non-canon view that she got out of there, returned to Gallifrey and became Lady President. She, too, would presumably have regenerated by now. Unlikely suspects, I think, would be the Doctor's grand-daughter, Susan. She may have been from Gallifrey, but it was never clear if she was part human and she never went through the Academy and therefore wasn't a Time Lady. Though we don't know what ever happened to her. I am really hoping it's either the Rani or Romana. The Rani would be a better plot choice because she and the Doctor - last of their kind - are unlikely to get along.
Ardal O'Hanlon (you may know him from the Brit-Com series "My Hero" or "Father Ted") has a nice bit part in the episode under the heavy prosthetics of a male cat person named "Brannigan." I really hope we see him again in a future episode, but I suppose that depends on whether or not the Doctor and Mary revisit the year 5 billion again.