Whatever Mohan is up to, it's part of series nine's heightened twist on Shetland's usual investigations. Spies, secret experiments, X-Files-style cold opens in which Looney-Tunes locals have their brains turned into mashed potato after getting caught trespassing in the no-go zone... it's a long way from this crime drama's usual fare. Unless the lab turns out to be a red herring distracting us from a more domestic answer to the question of who killed Annie and Anton, Shetland is venturing onto new ground.

And so is DI Calder, who's formed an unexpected - to her, at least - attachment to little Noah Bett. Ruth's awkward interactions with Noah are rather sweet to watch, like eavesdropping on a shy kid trying to make a new pal. Contrasted with Tosh's calm, capable, adult approach to the boy, Ruth comes across more like his peer than an investigating officer. The bond she's slowly building with Noah may prove vital if he remains the only witness to the murders.

That fact hasn't escaped Professor Rossi, the former Oxford spy - apologies, analyst - who appears to be running his own investigation into Annie's murder. Rossi has repeatedly asked after Noah, and this week parked outside his aunt's house to spy on his police visit. If the previous series hadn't revolved around a genealogy twist, we might assume that Rossi is Noah's biological father, but Shetland wouldn't repeat itself so soon. Rossi's interest in the boy must be down to his status as a witness. Does Rossi suspect Noah, does he want to know what Noah saw, or does he want to ensure that he keeps quiet?

In short: is the professor to be trusted? At the halfway point of a six-part murder mystery, it's traditional for everybody in the cast to look a bit sus, and Rossi's sneaking around this episode and turning up unexpectedly at Ruth's home (nice, the little nod to her continuing grief over Cal in that record collection chat), certainly fulfilled that requirement. He wasn't the only one dinging alarm bells. Debtor Patrick Harris and his creditor JJ Huang were clearly involved in some scheme that went awry on the night of the murders.

There could be one more of those to add to the list, depending on whether Patrick's younger brother Fergus pulls through after that cliffhanger argument on the boat. The Harris family are already a tragic bunch, and John accidentally killing his youngest only adds more pain. Poor Fergus was the sweetest of that already-sorry lot.

Back to the suspects: Frank also looked decidedly green when Tosh mentioned having found Annie's husband Ian at the seedy Mission during that awkward night out on fizzy water and dirty looks at the pub. He's hiding something. As are the wealthy Jakobsons in their magazine-spread home and expensive knitwear. Astrid Jakobson's phone call to Anton on the night he died was almost certainly not about the finer points of the US high school movie genre.

Clueless? Not quite. Tosh and Ruth have plenty to go on, but there are currently too many potential directions for a clear way forward to have emerged - which, at the midway point, is exactly where things should be.