The Better Mousetrap is the fifth book in Tom Holt's series that began with The Portable Door. The first four books told the adventures of Paul Carpenter, a fairly boring nobody who joined the firm of J W Wells & Co, not knowing its business is all magic and fantasy. Paul ends up the possessor of the Acme Portable Door, a unique item that fits inside a cardboard tube and rolls out a door, providing convenient access across space and time and dimensions.

In the fifth book, Paul has retired and his son Frank has inherited the Portable Door. Frank uses the door to help an insurance company executive avoid monstrous settlements. For 10% of the payment, Frank goes back in time and prevents the accident in question. His method involves extensive mathematical calculations, referred to several times and sort of spelled out, in order to identify to fix history properly, much like Asimov did in The End of Eternity.

But one accident, a simple cat stuck up a tree rescue gone awry, refuses to be fixed. Frank's calculations refuse to work when magic is involved, and he finds himself in an escalating war of fix and counterfix, neither side knowing quite who, where, or what the other side is.

As usual with Tom Holt, the book's humor is finely balanced and brilliantly inventive.

The Better Mousetrap can be read independently of the previous four books (which should be read in order).