Immortality can change a person. “Doctor Who” Season 9, episode 5 introduced Ashildre (Maisie Williams), a Viking who died trying to save her village. The Doctor (Peter Capaldi) brought her back to life, but he had to make her immortal. In episode 6, titled “The Woman Who Lived,” the Doctor and Clara (Jenna Coleman) will catch up with her a few hundred years later.

The Doctor and Clara might not seek out Ashildre. They head to 1651 to stop a criminal. “The deadly Highwayman ‘The Knightmare’ and his sidekick stalk the dark streets of London,” the synopsis teases. Of course, they aren’t just stealing candlesticks. They find something alien that causes the Doctor to interfere.

“Who is the Knightmare in league with? And can the Doctor avoid the hangman’s noose and protect Earth from a devilish betrayal?” the synopsis asks. While it isn’t clear if the Knightmare is teaming up with her, Ashildre is somehow connected to the crimes. The teaser trailer shows the Doctor running into her, and she’s a changed woman.

“I don’t need your help, Doctor,” Ashildre says. “You need mine.”

Watch the trailer for “Doctor Who” Season 9, episode 6 below:

“The Woman Who Lived” will not only reveal how she is different, but it will also show if she ever found a companion. When the Doctor made her immortal, he gave her the technology to give one other person eternal life so she wouldn’t have to watch everyone she loved die. Whovians will have to watch to see if she ever loved a person she couldn’t stand to lose.

“Doctor Who” Season 9, episode 6 airs Saturday at 9 p.m. EDT on BBC America.