‘Me and a conscious universe masquerading as a frog: BFFs.’ What a weird episode. Half a meditation on grief and loss featuring Bradley Walsh’s best performance to date (and his most Brian Williams, with emergency sandwiches) and some of the most surreal images the show’s ever offered up. Half a standard runaround some caves that wouldn’t have looked out of place in 1977.

The opening third seems to be leading in a different direction: the creepy house by a lake in the middle of a forest recalls the 1958 Norwegian film Lake of the Dead, with overtones of folk horror and sense of bleak isolation. Discovering Hanne boarded up, alone, in a house with a mirror that shows no reflection sounds like the set-up for a story that ends with everyone realising they’re already dead. Instead, the mirror, like those in Warriors’ Gate, leads into another space.

Sadly, it’s the Antizone, which has none of the strangeness of Warriors’ Gate’s black and white backdrops. It’s just some boring sets with Kevin Eldon in the thankless role of Ribbons, fleeing from some flesh moths. I found these stretches interminable, and a distraction from the thrust of the episode, there simply to thin the crowd of regulars and put Ryan in peril so Graham has a reason to go back. The solution seems simple to me – instead of the caves, have something from the mirror universe escaping into ours. Running away from monsters in some gloomy Norwegian forests would at least have given some scale and visual style to this slightly mechanical B-plot.

Luckily, if you haven’t Antizoned out, the mirror world beyond it is where the real heart of the story lies. There, the Doctor and Graham discover Hanne’s absent father Erik living with her dead mother Trine: he has abandoned his daughter and his life for happier memories, as good a metaphor for grief and withdrawal as any, leading to the powerful moment Hanne tells him, ‘You’re not well’. These sections are great: Erik threatening the Doctor with a rolling pin as she flips into Yaz mode, investigating and joining the dots. Whittaker is at her best in this setting, playing the Doctor like David Tennant’s Broadchurch investigator instead of his swaggering Time Lord as she pushes relentlessly towards the truth.

But the main event is Graham’s reunion with Grace. It’s beautifully done, exploring their relationship in ways the show has only previously touched on, and Walsh has to go through the curve of shock, denial and the possibility of a new beginning while the Doctor remains brutally truthful: ‘She’s not your wife, she’s furniture with a pulse.’ The depth of resignation and despair Walsh pours into his decision, ‘You’re a fake’, is magnificent.

And then they’re all sent home, and the Doctor is left with a talking frog on a chair. It’s absurd, but it’s meant to be. It ‘is a form that delights me’ and goes a long way to papering over my disapproval of the garbled explanation of the Solitract, and the fact it’s something the Doctor just knows rather than discovers. It’s definitely not what I was expecting after that atmospheric Norwegian opening. This story is all over the place, and some of it’s quite boring, but I did like the frog on the chair.

Next Time: The Battle of Ranskoor Av Kolos