Who's That Girl! was the Doctor Who back-up strip which appeared in the pages of The Incredible Hulk Presents #8-9. Like all in the series, it was monochromatic and featured the Seventh Doctor travelling alone at the beginning and ending of the story.
The story was unusual in several respects. It introduced a female Doctor in the form of Kasgi, an assassin who overpowered the Seventh Doctor to assume the "Doctor" identity as part of an assassination plan, and became invested in the part. The idea that "the Doctor" was a title, which other individuals than the Time Lord could embody, would later be revisited at greater lengths following the 2005 revival of the TV series, most notably with Clara Oswald in the 2014 episode Flatline.
It also featured the Seventh Doctor acting as a diplomat for Gallifrey seemingly on his own accord, and the idea of an alternate dimension permanently "syncing its timelines" with the prime universe to essentially merge with the Web of Time of its own accord.
An intergalactic treaty is about to be signed which will give the Frovians free access to the Kollian Dimension. The Doctor arrives in support of one of the signatories, the Frovian Prince Luj. But why is the Doctor now a woman? And just how much can the Doctor trust his royal friend?
In his palace on the planet Okul, Prince Luj is reminded by one of his advisors that many worlds in the "contested" Kollian Dimension have put bounties on his head, but Luj is insistent on greeting the Doctor personally when they arrive to the intergalactic diplomatic meeting as the representative of Gallifrey. Indeed, Luj enters the hall just as the Doctor's TARDIS materialises. But instead of the Seventh Doctor, the Doctor who steps out is a tall, blond woman wearing a long coat and striped trousers, who explains that she has recently regenerated.
Luj is initially startled at the the thought that regeneration can cause a Time Lord to change phyical sex, but gets used to the idea within moments, greeting the new Doctor as his old friend (though he briefly teases her with the thought that he might gain a romantic interest in her in this new incarnation). Luj then introduces the Doctor to the other delegates, such as Ux, the Fusian. Ux explains that he and the many other diplomats represent natives of the Kollian Dimension who believe that Luj's people, the Frovians, intend to carry out bloothirsty wars of conquest as soon as a smiling demeanour have brokered them entry into the Kollian Dimension.
Luj appears offended at the accusations of duplicity. The Doctor makes herself the arbiter, getting down on her knees and explaining that although the Kollian natives' worries are understandable, the Time Lords of Gallifrey have pledged to monitor the agreement and ensure that the Frovians remain peaceful. Luj is outraged at the Doctor humbling herself beneath the Kollians, whom he describes as "fools", and he shouts for the voting to begin at once. One of the Kollian emissaries seizes this chance to attempt to shoot Luj in an effort to prevent the warlike future they fear; however, the Doctor tackles them to the ground, knocking them out. Luj, impressed with her "moves", calls for the meeting to be adjourned.
As she walks back into the TARDIS, the Doctor is accosted by Ux who asks why she did not let the Kollian kill Luj. With a smirk, she replies that if she had done so, rather than wait to assassinate him herself, then she would have missed out on the money Ux will pay her for killing Luj. As the two enter the TARDIS control room, they find a dejected Seventh Doctor tied to the TARDIS control console with magnetic clamps. The woman boasts that she has made "quite a good him" and taunts him with the knowledge that he shall be wanted for Luj's murder by day's end.
The Seventh Doctor angrily asks the woman who she is. She insists that she is in fact the new Doctor, refusing to give any other name or explanation, but Ux addresses her as "Kasgi", letting it slip that he hired her to kill Luj. The Doctor insists the "cheapness" of her apparent occupatioon — assassin — but she retorts that, firstly, she is "very expensive", and secondly, she only kills people who she deems deserving of death. A wistful Seventh Doctor asks why, then, she has set her eyes on Luj, whom he knows as a noble soul and a friend. The Kasgi Doctor is nothing but amused at being antagonised by her "predecessor" and reveals that Luj is in fact the "two-faced, militaristic butcher" the Kollians believe him to be.
The Seventh Doctor, left alone in the TARDIS as Kasgi and Ux go out to arrange for the actual assassination, realises that even if Kasgi is right, the death of Luj would only precipitate a war between the rest of the Frovians and the Kollian Dimension. By reversing the polarity of a "powerful electro-magnet" built into the main control column of the TARDIS control console, he manages to counteract the magnetism of the magnetic clamps, freeing himself. Escaping, he makes his way to a secret passage Luj showed him on one of their past meetings which leads directly to Luj's private quarters. However, once there, before revealing himself, he overhears Luj gloating to his attendants about his impending enslavement of the Kollian Dimension.
A little later, the Seventh Doctor bursts into the meeting room just as the delegates are preparing to sign the treaty. He loudly declares that Gallifrey is withdrawing all support from the treaty and denouncing the Frovians. Realising they were almost tricked, the Kollians tear apart the treaty. Enraged but powerless, Luj swears to "have the Doctor's head for this". Meanwhile, Ux turns to Kasgi to formally announce that their contract is void — only to find that she has already vanished.
Having discarded her Doctor persona and outfit, Kasgi watches from a cornice as the original Doctor returns to his TARDIS. She feels pity for his having lost a friend in Luj, and respect for the way he managed to avoid the war entirely without spilling any blood — but as the calculating, assassin part of her mind becomes dominant once more, she begins to wonder if Luj really meant what he said aboout wanting the Doctor dead, and whether this might constitute a job opportunity for her...
- Luj claims to have known at least two of the Doctor's past selves (presumably including the Seventh Doctor).
- Kasgi's impersonation of the Doctor includes her dressing in the Third Doctor's frilly shirt, the Fourth Doctor's scarf, the Fifth Doctor's coat, the Sixth Doctor's trousers and the Seventh Doctor's pullover.
- It is never made clear exactly how Kasgi caught the Seventh Doctor and strapped him to the TARDIS console. He is simply shown as bound at the cliffhanger of part one.
- Luj is surprised to discover that regeneration can permit a change of sex and gender. Although the Doctor he is faced with is not actually a female regeneration of the Seventh Doctor, later accounts confirmed that this was indeed possible, for Time Lords in general (TV: The Doctor's Wife, Dark Water) and for the Doctor themselves. (TV: The Curse of Fatal Death, The Woman Who Fell to Earth)