Nadia Sandor (Mai Zetterling), an East European scientist, has defected to the British. The authorities are having trouble establishing her identity though and ask Drake to travel to Slavosk in order to free Nadia’s sister, Gerda (Barbara Murray), from prison and bring her back to England. But after he does, the problems aren’t over ….

Once again Drake is summoned to the presence of Hardy (Richard Wattis). Nattily attired in a three-piece suit (Drake’s English-wear?) our Secret Agent is still a little frazzled after his plane trip from America and (as is his wont) begins to rile the straight-laced Hardy. At one point Drake reveals that his friends call him “the man with the built in crystal ball”, which is something of a conversation stopper.

Drake’s voice over makes a comeback in this episode. It’s easy to see why, as it papers over the cracks when the narrative is forced to take a sudden jump forward. As touched upon before, that’s a curse of the 25 minute format – time is always of the essence.

With the assistance of Mikhail Radek (Sydney Tafler), Drake breaks Gerda out of prison. Radek is quickly established (via Drake’s voice over) as an amusing, if cold-hearted, mercenary – someone whose only loyalty is to money. To be honest, Drake lays this character profile on so thick that when Radek disappears from the story after a few minutes it’s hard not to imagine he’ll reappear towards the end. Guess what ….

The prison break could have easily lasted a whole act, but instead it’s done and dusted in a matter of minutes. Drake and Radek waylay the guards sent to escort Gerda to another prison and steal their authorisation documents. The hapless guards are dealt with in an amusing way though – lured by the prospect of girls and all-night jiving (the party they stumble into looks endearingly innocent) they instead find Drake and Radek waiting for them behind the bedroom door.

Given that the episode is now whipping along at a rate of knots, we never see the scene where Gerda realises that Drake is her saviour rather than another jailor. Instead, we have to be content with a single scene (on a studio-bound grassy knoll) which shows the pair leaving the country (thanks to Drake clipping through a barbed wire fence and having an energetic punch up with a guard).

This scene is notable for the way that Gerda, inching along the ground to the fence, stops to have a cup of tea from a flask whilst Drake is attempting to break through. Now I like a cup of tea as much as the next man or woman, but surely there’s a time and a place to take your beverages.

Is Nadia an imposter? The arrival of Gerda should hopefully answer this question, but since the sisters haven’t seen each other since childhood that won’t be so easy. Plus there’s the very real possibility Gerda could be a spy sent to discredit Nadia and force the British to deport her.

This is a decent puzzler and both Zetterling and Murray play the scenes they share together well – the sister’s reactions abruptly changing from delight as they’re reunited after many years apart to suspicion as each apparently begins to mistrust the other.

If one were in nit-picking mode, then it’s slightly hard to believe that no-one has been able to vouch for Nadia Sandor. No doubt she’s rarely travelled abroad, but given that she’s fairly eminent in her field, would she never have been photographed in the newspapers or met any Western scientists?

As it turns out, Gerda is revealed to be a spy which secures Nadia’s place in Britain. But maybe a more devious writer would have ensured they were both imposters, with the exposure of one as a spy helping to strengthen the identity of the other.

The ‘shock’ late return of Radek, working with Gerda, helps to wrap things up. Gerda pleads with Drake not to send her back home, but Drake is implacable. We never learn her fate (or that of Radek) but Drake tells her that “when you play this sort of game, you must expect to pay the consequences”. Ouch.

In addition to Zetterling, Murray and Tafler, the always reliable Anthony Dawson makes a brief appearance, meaning that The Sisters doesn’t skimp on acting talent. It might be another of those episodes that really could have breathed had it had double the running time, but it’s still an above average effort.