Mayfield's Law

When Superintendent John Mayfield took Harriet to the theatre, it should have been a pleasant night out for both of them. Instead, Harriet quarrelled with John, left Warwick to go back to Australia, and did not intend to return.
Struggling to accept her departure, John began seeing Laura Grant, a new schoolmistress in the town. Whilst investigating the suspicious death of a local bank manager, John first meets Richard Gilson, the new doctor, unaware he is also keen to meet Laura. Soon afterwards the bank is robbed, and a murder is reported. Are the two crimes connected?
Two other deaths occur, and John now knows Dr Gilson is seeing Laura, but he has to work with him. Whilst this all happening, someone is poisoning John's friend, Dr Thomas Waldren, and the main suspect is his wife, Sarah Waldren.