The Doctor and crew land in Norway and find a young blind girl named Hanne alone in a boarded-up house. She has not known where her father was for several days and is afraid of a monster she believes is outside her house.
In the attic is a mirror that is a portal to another world. When they step through the mirror, they find themselves in an Anti-Zone (a buffer-space between universes to prevent catastrophic damage). The mirror universe on the other side is a sentient universe known as the Solitract plane. It was originally part of our own universe but was ejected at the dawn of time due to incompatibility with the rest of the universe.
The sentient universe is benevolent; however, its desire to befriend us is contrary to the threat it poses us. It entices people by creating doppelgangers of lost loved ones. They find Hanne’s father, Erik, on the other side, and he believes his late wife has returned. Graham also believes that his own wife has returned as he sees Grace. The Doctor does not see anyone from her past (as would be logical); instead, she sees a frog because Grace liked frogs. This part felt a bit sloppy. The first two had people from their past come back, but it almost seems like they did not know how to have the Solitract connect with the Doctor in a meaningful way, so they just had it appear as a frog.
The Doctor is eventually able to convince the Solitract to let them go, because their incompatibility will cause them all to die. It is always refreshing to have the occasional optimistic resolution where the Doctor saves the day by talking instead of resorting to violence.
The problem is that while the universe is not really a monster, the actual villain of the episode gets away without any consequences. Erik is a terrible person and an abusive father, yet the Doctor acts like he has learned his lesson without actually addressing what he did wrong. When his wife died, he moved with Hanne to be isolated in the middle of the woods. It turns out that the monster she fears was just made up. He hid speakers in the woods to project monster noises to keep her inside. He says it is to protect her from bears, but we never see any bears nor do we see any evidence that they are around. Then, once he goes through the portal, he immediately abandons Hanne. I understand his desire to be reunited with his wife, but he never even tries to go back to bring his daughter with him.
Thankfully, to counter this toxic family dynamic, we actually see a healthy one, and it was one of the nicest moments all season. Graham has been desperately seeking Ryan’s approval all season, and the two have both been dealing with their grief at the loss of Grace. They realize that the way for them to heal is to do it together. In the final moments, Graham finally gets his wish to hear Ryan refer to him as “Grandad,” and this touching moment was needed to help get past how terrible Erik is as a father.