This is an enjoyable novellet. That said, I couldn't help assuming it was a companion to another series. After finishing the story, I discovered it was just that. The Paratime Police are a group of law enforces for parallel timeline jumpers.
In Police Operation, a traveller takes their cosmic space-pet into a primitive timeline, like ours, and ends up dead, releasing their space-pet into said timeline's wilderness. Needless to say, our protagonist, Verkan Vall, sets out on a monster hunt!
Originally published in 1948, it contains a lot of the sci-fi themes you'd expect from that era. Verkan Vall is a bad-arse, lady-killer of a cop: a real man's man. There are also quite a few references to the primitive (called fourth-level) timeline and the industrial progression during war. It also had a lot of info-dump sessions. As I said, it felt like a companion story. A good chunk of this story involved a rocket ride, in which Verkan Vall explained exactly how the Paratime Police jump timelines. After that, he went through another big info dump session on morales etc. with his boss. It had the feel of all that theoretical stuff those earlier sci-fi books loved to throw around.
The hunt itself was kinda cool. The idea of the poisonous cockroaches, and the jacket, felt both silly yet interesting at the same time. I enjoyed it. The monster was a classic space-monster, which I envisioned as a skinny and tall version of angry Stitch (Lilo and Stitch) brought to life with old B&W Doctor Who special effects.
Police Operation is a little corny, but still enjoyable. I'm open to hooking into more Paratime Police adventures!