The story is written in such a way that much of the action occurs "offstage," so to speak, and can only be imagined by the reader based on the behavior of Alan Austen and the words of the mysterious old shopkeeper.
Austen is in love with a girl named, appropriately, Diana, after the beautiful young goddess who refused to have anything to do with men. In desperation Austen has come to the mysterious, and wholly improbable, shop where the wise old proprietor sells love potions. We can only imagine what has occurred between Austen and Diana to drive him to this expedient. He wants to buy a love potion. When he leaves he will slip it into Diana's drink, and she will fall madly in love with him, as the old shopkeeper foretells. This old man warns him that he will find that Diana's love is too overwhelming. She will cling to him. He will lose his freedom. But Austen doesn't believe that could happen, since in his present emotional state he would gladly be bound to Diana for the rest of his life.
The rest of the story does not have to be told in words. Austen exits with the love potion, having been advised that a "chaser" is also available in case he should need it in the future. The reader can imagine the outcome. Diana becomes clinging, possessive, demanding. Austen begins to hate her. He will return to the shop and pay a stiff price for a "chaser," which is actually an undetectable poison, and dispose of Diana forever.
The rising action occurs "offstage." Austen falls in love with Diana and tries unsuccessfully to woo her. The more he pursues her, the more she rejects him. He learns about the shop that sells love potions and buys a bottle. We can guess the interactions between Austen and Diana from the state he is in when he enters the shop.
The falling action occurs "offstage." Diana falls madly in love with Austen and becomes too possessive.
The resolution occurs "offstage." Austen returns to the shop, buys the "chaser," kills his wife, and avoids punishment because the poison is undetectable.
By telling his story in such a way, Collier achieves an Aristotelian unity of time, place and action. He also spares the reader the grisly details of the murder.