Season 11, Episodes 11-12

Not a two-parter - more like a diptych ("the Dark Diptych"?):

Episode 11: Shadow Play

After a brief stay in Avalon, Lady Penelope must honour a sad appointment with history - the funeral of her dear old friend Sir Lawrence Stapleton, aka Uncle Larry, former director of Torchwood and her first-ever time travel companion. But in the corridors of power, a conspiracy is being hatched against the great man’s legacy and history is being manipulated by an invisible hand… A tale of hidden agendas, dark designs and unseen enemies.

Episode 12: Gaze of the Abyss

Her hunt for the Millington Entity takes Penelope to the East End of London in 1896, where an all-too familiar monster has once again begun to walk the streets, sowing death and terror in its wake - but as the Time Lady well knows, Darkness comes in many shapes, not all of them faceless. As the Dark Dimension looms in, Time is, once again, of the essence.

Notes: So Who (or What) Was the Millington Entity, Anyway?  

The Entity was the ultimate incarnation of a character whom Penelope had first met in Season 7 (in Episode 7.8: Darkness Ex Machina) back when she was battling the returned Fenric; she started life as Professor Andrea Millington, cold-hearted scientific genius, daughter to the Commander Millington from The Curse of Fenric) and one of the "Wolves of Fenric" chosen by their masters to engineer His Triumphant Return in various time periods (in this case, the 1990s). This dark design was foiled by Lady Penelope, whose interference caused a "big infernal device disaster" which left Professor Millington crippled - yes, this was, of course, a weird, cosmic (or meta-narrative?) wink at Dr. Judson's condition in Curse of Fenric. 

Professor Millington returned in Episode 2 of Season 9 (set in 2015 or so) - now working as a scientific adviser for UNIT but with a sinister hidden agenda of her own (to keep things as brief as possible: get a new, healthy and potentially immortal body by stealing Penelope's life-force). At the end of this episode, Professor Millington was a prisoner of UNIT - a bit like the Master in the 1970s but at least she was still (biologically) human. She waited for the time of her revenge (and so did the game master)... 

Shadow Play (see above) saw her return in a new role and condition (the mysterious "invisible hand" mentioned in the blurb), that of a psychic menace using the "bloodline network" of the Wolves of Fenric to manipulate events and engineer her own rebirth / apotheosis as a kind of successor to Fenric, a new entity tied to the entropic infinity of the Dark Dimension. 

Luckily for the universe (and history as we know it), this plan was once again foiled by Lady Penelope, who had to pursue the disembodied Millington Entity in time, back to the 1890s, where she took possession of one of her ancestors, a time-sensitive working for Torchwood in the East End of London... and tried to awaken "the Wound", a dimensional rift which gave access (among other things) to the Dark Dimension and which had played a significant role in some of Penelope's early adventures (see The Shadow Below London, Episode 9 of our first season). Penelope could not save the possessed ancestor (who was pretty much doomed the moment the Millington Entity took possession of her) but managed to trap the nascent Fenric wannabe in a specially-crafted temporal stasis, where the dark, entropic energies of the Entity eventually devoured her own remaining psyche, reducing her to just another cloud of mindless Dark Energy...