Karabraxos directs Ms. Delphox to have the Teller brought to the vault, before having Delphox incinerated for letting Karabraxos down. Before Karabraxos departs, the Doctor gives her the TARDIS phone number to use when she has a moment of regret. When the Teller arrives, the Doctor lets it restore his short-term memory. The call to the TARDIS was from a dying Karabraxos, regretting something she left in the vault. The Doctor became the Architect of the time heist to assemble Saibra and Psi and plant the teleports to get them to this point. The Doctor shows them why they had six teleports: Karabraxos wanted him to rescue the Teller's mate that she had locked up to force the Teller's cooperation. The Teller frees its mate. The two are evacuated to an unpopulated planet to live out in peace. Memory worms were first introduced in the 2012 Christmas special, "The Snowmen". When Psi tries to lure the Teller away from Clara by visualising some of the galaxy's most notorious criminals, the mugshots he is seen accessing include a Sensorite (from the 1964 serial "The Sensorites"), a Terileptil (from the 1982 serial "The Visitation"), a Slitheen, an Ice Warrior, the Gunslinger from "A Town Called Mercy", Captain John Hart from "Torchwood", Androvax and the Trickster from "The Sarah Jane Adventures", and Abslom Daak from "Doctor Who Magazine". Speaking about the premise for the episode, director Douglas Mackinnon said “What we wanted to do was a heist movie for "Doctor Who". I've watched virtually every heist movie there's ever been, and it incorporates things into it, but because it's "Doctor Who", time travel is involved." The read-through for the episode took place on 11 February 2014. Filming began on 3 March 2014. Filming took place at George Street in Bridgend on 5 March 2014, and in Roald Dahl Plass, Cardiff Bay on 13 March. Filming continued at the Hadyn Ellis Building, part of Cardiff University, on 18 March 2014, and at the nearby Bute Park the next day. Media coverage of filming made note of a new monster that was spotted on set, with some calling it "Doctor Who"'s "strangest monster yet." Scenes were also filmed at the Uskmouth Power Station, previously a location for "The Doctor, the Widow and the Wardrobe" and "Into the Dalek".