The Kasaavin are the main antagonists of Doctor Who Series 12 opener Spyfall, acting as inter-dimensional spies, looking to take over the main universe. They were voiced by Struan Rodger.
The Kasaavin are beings that come from a realm beyond N-Space. However, they spent years on Earth as sleeper agents, planning to take the planet's "universe". Sometime before 2020, the Master discovered them and formed an alliance with them along with Daniel Barton. He planned to use them to convert the entire planet into hard drives, although he planned to betray them later. The Kasaavin abducted several human spies and altered their DNA to no longer be human.
When the Doctor and Graham went to see the Master (posing as O), who was living in the Australian outback. While there, the Kasaavin attacked them, and simultaneously in VOR HQ, some Kasaavin met with Barton, and noticed Ryan and Yaz, abducting her. When the Doctor managed to trap one, it managed to escape by switching places with Yaz. When the Doctor and her companions suspected of Barton being responsible for the Kasaavin attacks, he angrily denied the allegations and left in his car where Team TARDIS chased after him to his private airport. While on board, O revealed he was the Master and was responsible for the Kaasavin attacks all along. The Master then teleported to his Tardis while the Kassavin sent the Doctor to their Dimension.
While in the Kasaavin's dimension, the Doctor met Ada Lovelace and both were teleported to London in 1834. There, they faced off against The Master until Ada threw a shotgun ball at the Master, forcing him to flee. At Ada's house, the Doctor tried to use The Silver Lady, a device that can summon the Kassavin made by the Master to get back to 2020, Ada grabbed her hand at the last second.
They were then sent to Paris in 1943 instead, where they meet the British spy Noor. After meeting with The Master on top of the Eiffel Tower, the Doctor learned the Master sent the Kaasavin through different time periods to gain information from people who were important to the development of computers. The Doctor with the help of Noor, disabled the Master's prescription filter, causing him to be arrested by the Nazis. They then used The Master's TARDIS to get back to 2020.
Meanwhile, back in the present, Daniel Barton had the Kasaavin kill his mother after she sarcastically praised his work. The Doctor, Ada and Noor went into a warehouse where her companions had stayed overnight in order to stop the Silver Lady from activating, before then the Master came inside the warehouse after living on Earth for seventy-seven years. However, the Silver Lady activated and started to paralyze everyone, until it suddenly failed.
The Doctor then revealed she planted a virus in the bomb in mid-2019 and played a recording of the Master revealing how he truly feels about the Kasaavin and revealing his intent to betray them. Enraged, the Kasaavin imprisoned the Master in their dimension. Later on, the Doctor tried to visit their dimension, while looking to see if he escaped only for it to be inaccessible by the TARDIS. Eventually, the Master would escape their dimension.
- The Kasaavin somewhat resemble the Vardans from The Invasion of Time, with both being energy beings that can transport themselves anywhere.