Magic's Doctor Who crossover takes inspiration from across the show's history. There are sorcery cards for catchphrases like Reverse the Polarity and Everybody Lives; saga cards that let you recreate episodes like An Unearthly Child and The Girl in the Fireplace; and cards for characters like Jamie McCrimmon, the Second Doctor's highlander companion with a penchant for leaping into battle (appropriately, he has Trample), and Karvanista, the hyper-evolved dog who fought alongside the Thirteenth Doctor.
The Karvanista card emulates the Lupari species' protectiveness toward humans—man's best friend and all that—by letting you either cast him to make cards with the human type indestructible for a turn, or summon him and give all your humans a +1/+1 counter. He's got Trample as well, since he's just as likely to leap in swinging as Jamie was back in the day.
Rather than being a mainline expansion, Magic's Doctor Who cards are just for the Commander format. They include four pre-made decks, one collecting the show's villains, and three themed around different eras of the Doctor. The Blast From the Past deck features the first eight Doctors (and is where you'll find characters like Ace, Fearless Rebel, who powers up when you blow up Artifact cards), while Timey-Wimey is the reboot deck with Doctors Nine through Eleven (the Farewell card, depicting David Tennant's departure, should come pre-stained with tears). Paradox Power covers the most recent incarnations, Twelve and Thirteen (where you'll find Karvanista, as well as a saga card that lets you recreate Heaven Sent, the episode where Peter Capaldi was trapped in a castle right out of M.C. Escher).
The big new mechanic for this crossover is Time Travel. Some cards can be Suspended, which means instead of playing them immediately you put them to one side under a stack of time counters. One time counter gets removed every turn, and once they're all gone the card finally gets played. Cards with Time Travel effects, like the delightfully named "Wibbly-wobbly, Timey-wimey", let you remove time counters to bring Suspended cards into play sooner, or add time counters if there's a reason to delay them.
For a show about someone who owns a time machine, Doctor Who is rarely about time travel. The Doctor uses time travel like you should use alcohol—never to solve your problems, only to create new and more interesting ones. When time travel does factor into the plot, it's usually a way of adding a sense of inevitability, like a whole season of references to "Bad Wolf" in different eras before we find out what it means, or Amy Pond meeting the Doctor repeatedly throughout her life before finally becoming a companion. That's what the Time Travel mechanic emphasizes, rather than just being a way to counter effects or something.
That's what Reverse the Polarity is for.
The Doctor Who x Magic: The Gathering cards will be available from October 13. You can try them out by attending a Commander launch party at your participating friendly local game store on the release weekend.