Bradley Walsh and Tosin Cole will make their final appearances as the Doctor's companions in the Doctor Who special on New Year's Day, the BBC has revealed.
Walsh said viewers should "expect a lot of poignancy" from the episode.
The pair have been at Jodie Whittaker's side since 2018. Cole said it had been "an honour" to play Ryan opposite Walsh's Graham and Whittaker's Doctor.
Titled Revolution of the Daleks, the special will also see John Barrowman return as Captain Jack Harkness.
Barrowman said it was "great being back" and that returning to the show had been "like going home".
Walsh and Cole made their Doctor Who debuts at the start of Whittaker's first full series as the time-travelling Time Lord.
They went on to appear in two series as well as the 2019 special Resolution, which also featured the villainous Daleks.
Walsh said it was "amazing" to be "only one of a few people on the planet" to have battled the Doctor's long-time robotic nemeses.
The comedian and host of game show The Chase said he had "absolutely loved" being on the long-running sci-fi programme and would miss "everyone and everything" involved.
Meanwhile, Cole said filming his last scene had been "emotional".
Barrowman, who has played Captain Jack in both Doctor Who and its spin-off Torchwood, said the Daleks were the show's "quintessential" villains.
"They are never to be underestimated as they will always find a way to survive, which is exactly why they have survived over centuries," he added.
Cole said: "Working with the Daleks is sort of like working with Doctor Who royalty. You have to respect them because they are so iconic."
The Christmas special will see Mandip Gill resume her role as companion Yasmin, while Chris Noth returns as business tycoon Jack Robertson.
It will also see Dame Harriet Walter make her Doctor Who debut alongside Nathan Stewart-Jarrett, of Utopia and Misfits fame.
"We've crammed this year's Doctor Who festive special with an explosion of extraordinary acting talent," executive producer Chris Chibnall said.