Joan visits Elana March in prison. March denies hiring the assassin who put hemlock in Joan's drink, but comments that she heard Joan had been breaking up with Andrew when he died.
Gregson says March's comments don't prove she did it, but he is assigning uniforms to watch Joan's apartment. Sherlock tells Joan to let him know when she is ready to start working again.
Some pregnant zebras have been stolen from the Bronx Zoo. Sherlock calls Detective Bell to assist him with the case.
They determine the thieves stole a delivery truck to transport the zebras. Sherlock finds one of the original packages contained a GPS dog collar, which leads to them to the abandoned packages. Sherlock finds a slip of paper with an equine hospital's address.
Andrew's father drops off some of Joan's things from Andrew's apartment. She apologizes again. He doesn't blame her, but asked her why she waited to take precautions against March.
Sherlock and Bell find the abandoned delivery truck in an empty warehouse. They find the two zebras who were given a drug to induce labor. Their babies are missing. Sherlock finds a dead body.
Bell speculates what happened and Sherlock corrects him. The victim, David Chang, was not a willing participant but was forced to help deliver the babies.
Sherlock visits Joan. She explains that she pulled away because she had wanted something of her own, which Sherlock understands. Joan wonders if Andrew would be better off if she hadn't.
They search Letchworth Village where they believe the zebras are being kept, but find a quagga instead. Quaggas have been extinct for more than a 100 years.
Bell and Sherlock share what they uncovered with Dr. Jospeh. He brings them to the zoo to meet with the staff who cared for the zebras. Sherlock accuses one of the staff of stealing the zebras and killing David Chang. It was a trick to relax the real culprit, Ben Reynolds.
They interrogate Reynolds and tell them they have the killer's DNA. He asks for a lawyer.
Sherlock visits Joan. She receives a postcard from Elana. Bell calls and the DNA matched Reynolds but he is in the wind.
Sherlock meets Bell at Reynold's home to figure out how he escaped. Sherlock finds a tunnel between Reynold's home and the former speakeasy across the street.
Reynolds only thing of value is the one quagga he still has. Sherlocks suspects he will try to auction the animal off so he intends to pose as a buyer.
Bell wakes up to find a note from Sherlock. He meets Sherlock at a cafe. They watch the police arrest Reynolds.
Joan receives a note from Moriarty saying that she is aware of the threat against her, which is unacceptable because Moriarty still has plans for Joan. Elana is found dead in her cell.
Joan tells Sherlock that she wants to commit to their work and move back into the brownstone.