The newly-regenerated War Doctor lands on Gallifrey and offers his fighting services to Rassilon. Once a war-changed Romana II and the Fourth Doctor show up, Dalek spies disguised as Time Lords reveal themselves to have infiltrated the capital. The four go on the run to escape the Daleks using the Omega 13 protocol and regroup to plan a re-seizure of their home city - through disguising themselves as the very Dalek spies they're fighting. ("One, Two, Three, Four: I Declare A Time War")
Some time after that, the War Doctor is sent by Rassilon after the Ivatoisli, a poisonous walking plant species that can prevent regeneration, whom the Daleks appear to be weaponizing. Upon finding one plant-creature murdered, with Darren Hughes to blame, the War Doctor unknowingly unites with his to-be-companion, Amy Pond, to escape the wrath of the Ivatoilsi's bretheren. ("Death's Door")
After returning from that mission, the Time Lord searches for anomalies in time that could be caused by hostile Dalek intervention. He comes across an eerily cold summer of the planet Earth, as well as his old friend Candalynne in her twelfth incarnation, the late Harley Quinn's sister Isabella, and the War Doctor's own tenth self. Here he learns of what the Valeyard did to separate the Doctor's timelines, allowing for discontinuities amongst each Doctor's adventures. ("The Cold Summer")
In the midst of battle against the Daleks and Iviatoisli, the War Doctor receives a letter from the Brigadier's estate, along with all the other Doctors, including the Fourth. Upon being summoned to the Brigadier's funeral, the War Doctor meets the ex-Time Agent Eric Tempore. ("Concerning the Death of Brigadier Alister Gordon Lethbridge-Stewart")