His beloved wife having died in childbirth, Phineas Finn finds Irish society and his job as a Poorhouse Inspector dull and unsatisfying after the excitement of his former career as a Member of Parliament. Back in England, the Whigs are determined to overturn the Tory majority in Parliament. As Finn had been considered the most promising of the younger set, he is encouraged to stand for parliament again.
Returning to London, he renews his acquaintance with the wealthy widow, Madame Max Goesler. In the past, she had offered to marry him and had been gently turned down; after an awkward first encounter, they renew their friendship.
In the political arena, Finn loses the election by a narrow margin, but his luck does not desert him. On appeal, it is found that his opponent had bribed some of the voters, enough to give Finn the victory. He does however make one enemy within his own party. Mr Bonteen makes disparaging remarks about his political trustworthiness (referring to an incident described in Phineas Finn). The conflict spirals out of control when neither man will back down, and they become bitter foes.
When Bonteen is murdered, suspicion falls on two men. One is the Reverend Mr Emilius, husband of Lady Eustace (the main character of The Eustace Diamonds). At her urging, Bonteen had discovered that Emilius had been married when he wed Lady Eustace, thus annulling the marriage and safeguarding her wealth. The other suspect is Phineas Finn. He and Bonteen had been seen to quarrel violently the night of the murder and all the circumstantial evidence points to him, while Emilius did not even have a key to exit his lodgings that night. Finn therefore is brought to trial. Not unexpectedly, the murder of one Member of Parliament allegedly by another quickly becomes the sensation of all England.
While the trial goes on, Madame Max travels to the Continent looking for evidence, and she succeeds. She finds a locksmith who had made a duplicate key for Emilius. This, along with other developments, convinces everyone that Finn is innocent and Emilius guilty. Unfortunately, it is not enough to convict the latter.
Afterwards, Finn, worn out by the ordeal and disillusioned with politics, refuses an invitation to take office in the government, and marries Madame Max.

Question: Why is Finn not convicted of killing Bonteen?
Answer: It is found that Emilius had a spare key made.
Alternative answer: Madame Max went looking for evidence.