Act I: The play opens with Agatha being ejected from an inn when her money runs out. Too proud to beg, she is desperate. Frederick enters, sees her, and offers her money although it will mean he cannot pay for his own breakfast. She recognizes him as her son, absent for five years as a soldier. He has returned in order to find his birth certificate, necessary in order to find employment. Distraught, Agatha tells him that there is no certificate: she was seduced at the age of seventeen by Baron Wildenhaim upon promise of marriage. Despite the resulting pregnancy, the Baron broke his promise and married another, wealthier woman, and Agatha, turned out of her home, struggled to make ends meet and raise her son alone. Frederick is dismayed at the news, and to find his mother so destitute, and he arranges for her to stay with some charitable cottagers.
Act II: Frederick leaves, intending to beg for money. Not knowing her relationship to him, the cottagers tell Agatha of the recent history of Baron Wildenhaim, now widowed and with a daughter. She faints.
Meanwhile, reluctant to force her inclination as his own was forced, the Baron tries to determine whether or not his daughter, Amelia, loves the affected and foppish Count Cassel. It becomes clear to the audience that she instead loves Anhalt, a poor clergyman, and he her. It also becomes clear that the Baron regrets the misdeeds of his youth and has long sought, fruitlessly, to make amends.
Act III: Frederick, desperate as his begging has been unsuccessful, attempts to rob the Baron and the Count as they go hunting, not knowing who they are. He is arrested.
Anhalt speaks to Amelia about matrimony, at the Baron's request, but she in turn confesses her love for him and forces his own confession. They are interrupted by Verdun, a versifying butler, with the news of Frederick's attack on the Baron. The Baron enters and Amelia pleads for the life of the unknown young man, but the Baron is adamant on the grounds that an example must be made.
Act IV: Amelia takes Frederick food and he discovers who it was that he had threatened. He asks to meet the Baron privately.
Amelia reveals to her father information she learnt from Verdun about the Count's sexual immorality. The Baron confronts the Count; he replies that he is a man of the world, and reminds the Baron that many men have behaved likewise. Confused and embarrassed, the Baron dismisses him; Amelia re-enters and reveals her love for Anhalt. They are interrupted by Anhalt, who tells the Baron that Frederick is in the next room and wants a private interview. Frederick reveals their relationship to the Baron, and then leaves. The Baron is much affected.
Act V: Anhalt goes to Agatha at the cottage and explains some circumstances which mitigate the Baron's previous conduct.
Frederick, and then Anhalt, insist to the Baron that he must marry Agatha. After some agitation because of the social differences between them, he agrees, and in gratitude to Anhalt for his advice, agrees also to let him marry Amelia despite his poverty. Agatha enters the room and all are reconciled.

Question: Why was Frederick arrested in Act III?
Answer: He tried to rob Baron and the Count.
Alternative answer: He attempted to rob the Baron