text: Tennant was joined by Kylie Minogue portraying Astrid Peth in the 2008 Christmas special, "Voyage of the Damned." This episode also introduced Bernard Cribbins as Wilfred Mott, later revealed to be Donna's grandfather, who would continue to recur across series four. Cribbins was not originally expected to appear outside of the special, but was asked to return in place of Howard Attfield who had portrayed Donna's dad Geoff and died unexpectedly while filming was still underway. Despite having another actress and character lined up for the fourth series, the producers ultimately decided to ask Tate to return instead. Piper returned for this series making a set of various cameo appearances. Ageyman also returned for three episodes mid-series. Alex Kingston was introduced as a mysterious time-traveler called River Song in the later portion of the series during which her character was killed-off. The series four two-part finale saw numerous cast members return including Piper, Barrowman, Agyeman, Sladen, and Leeson, while also featuring the departure of Tate. Tennant decided to leave the role following a series of specials which featured David Morrissey, Michelle Ryan, and Lindsay Duncan as one-time companions in their respective episodes. "The End of Time" saw the return of Cribbins, this time as the Doctor's companion. This episode also saw brief cameo appearances from Piper, Tate, Agyeman, Barrowman, and Sladen ahead of Tennant's regeneration.