Among all the twists and turns of this week’s Doctor Who, there was also a touching callback to one of the more enduring characters created by the modern era of the sci-fi series – Alex Kingston’s River Song, who made an intervention from beyond the grave to prevent the Doctor from executing his old frenemy Missy (Michelle Gomez). 

Reading from River’s trademark Tardis-patterned diary (presumably recovered from the library where the Doctor left it in 2008’s Forest of the Dead), Nardole (Matt Lucas) reminds the Doctor of his wife’s written words, encouraging him to think again before committing the deadly act. 

“Goodness is not goodness that seeks advantage. Good is good in the final hour,” he tells him. “In the deepest pit, without hope, without witness, without reward. Virtue is only virtue in extremis. 

“This is what he believes, and this is the reason above all, I love him. My husband. My madman in a box. My Doctor.”

More succinctly, he added: “Your missis wouldn’t approve.”

Nardole goes on to explain exactly how he tracked the Doctor down to his far-flung location (“Followed you from Darillium, on the explicit orders of your late wife River Song,” he says, referring to the trio's last onscreen appearance together in 2015's The Husbands of River Song), and it’s both a neat way to fill in some backstory (revealing how the Doctor and Nardole reunited as well as why they ended up guarding the vault) while also paying tribute to River’s influence on the Doctor’s life.

Still, we also can’t help wondering if it’s hinting at something more. Over the course of this series there’s been a whole host of references to River, from the obvious (the Doctor keeps a picture of her on his desk, above) to the subtle (the time-travellers return to a favourite holiday spot of the Doctor and River, and the Time Lord also reminisces about one of their aquatic mutual friends). 

One explanation for this is just that head writer Steven Moffat is feeling nostalgic for one of his most significant additions to Doctor Who canon (he created River for her first appearance in 2008) in his final year working on the show, foregrounding her importance to both the Doctor and the series lore in his final run of episodes.

On the other hand, it could be that this drip-drip-drip of callbacks is functioning as a primer or refresher for audiences before River comes back for a final TV appearance, reminding viewers of the character in subtle but frequent ways designed to make a reappearance less of a shock to the system (or risk alienating new/casual viewers).

Either scenario seems fairly plausible for Steven Moffat’s last ride – he’s previously said that The Husbands of River Song, which tied up the River/Doctor storyline, could have been his Doctor Who swansong – but whatever the truth, we’re sure fans will be keen to see how River will factor in to the remainder of the series. No spoilers, please. 

Doctor Who continues on BBC1 on Saturday 27th May at the later time of 7.45pm